The Ultimate Creamy Spinach Casserole with Crispy Onions

I’ll never forget my first real introduction to a proper potluck. It was shortly after we’d moved into our new neighborhood, and the ladies from the church auxiliary were hosting a welcome luncheon. The tables were groaning under the weight of every casserole imaginable, from broccoli and cheese to sweet potato with a praline topping.

But the one that caught my eye was a simple, creamy spinach casserole. It was comforting and familiar, the kind of side dish that feels like a warm hug. It was good, truly. Yet, as I took a bite, I couldn’t help but think it was missing a little something. A little bit of zing, a little bit of character.

That afternoon, I went home with that casserole on my mind. I knew the base was solid, but I wanted to make it mine. I wanted a version that people would ask for, the one that disappeared first at the potluck. After a bit of tinkering in my own kitchen, I found the secret. It was a little package of Boursin cheese. That creamy, herby, garlicky cheese was the pop of flavor it needed, turning a classic side into an unforgettable star. This is that recipe, my go-to, never-fails spinach casserole that always gets rave reviews.

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Why This Spinach Casserole Recipe Works

Spinach casserole is a cornerstone of Southern cooking and holiday tables everywhere. Its roots are in those creamy, baked vegetable dishes that became popular in the mid-20th century, think green bean casserole’s sophisticated cousin. It was a way to make vegetables feel indulgent and special, and it quickly became a potluck and Thanksgiving staple. Many versions rely on simple cream cheese or just mayonnaise and sour cream for their base, which is delicious but can sometimes taste a little one-note.

This recipe takes that beloved, comforting classic and elevates it with a few thoughtful ingredients. The real game-changer here is the Boursin Garlic & Fine Herbs cheese. Instead of just adding creaminess, it infuses the entire casserole with a perfectly balanced blend of garlic, parsley, and chives. You get all that complex, savory flavor without having to chop a single clove of garlic or measure out a bunch of dried herbs. It does all the heavy lifting for you, creating a depth of flavor that makes people wonder what your secret ingredient is.

We don’t stop there. The addition of nutty, salty Gruyère cheese provides a superior melt and a more complex flavor profile than standard cheddar. A whisper of fresh nutmeg enhances the earthiness of the spinach, and the crispy fried onions folded into the casserole and sprinkled on top add a delightful texture and savory crunch. It’s a symphony of textures and flavors, creamy, cheesy, and crunchy, that transforms a simple side into the star of the show. It’s familiar enough to feel like comfort food but special enough to impress your guests.

Ingredients for Spinach Casserole

The magic of this spinach casserole lies in using simple, high-quality ingredients that work together to create something truly special. Each component is chosen to build layers of flavor and texture, resulting in a dish that’s creamy, savory, and absolutely irresistible. The key is to let a few star ingredients shine without overcomplicating things. This is comfort food, after all, just with a little extra flair.

Here is exactly what you will need to gather from your pantry and refrigerator to get started. Don’t be tempted to skimp on the quality, as it really does make a difference in the final taste and texture of this crowd-pleasing side dish. The full-fat dairy and good cheese are what make it so rich and satisfying.

• 2 (10-ounce) packages frozen chopped spinach, thawed and squeezed very dry
• 1 (5.2-ounce) package Boursin Garlic & Fine Herbs cheese, softened to room temperature
• 4 ounces cream cheese, softened to room temperature
• 1 cup sour cream
• 1/2 cup mayonnaise
• 1 cup shredded Gruyère cheese, divided
• 1/2 cup grated Parmesan cheese
• 1 large egg, lightly beaten
• 1/4 cup crispy fried onions (plus 1 cup for topping)
• 1/4 teaspoon freshly grated nutmeg
• 1/4 teaspoon black pepper
• Pinch of salt

The true star here, my secret weapon, is the Boursin Garlic & Fine Herbs cheese. This isn’t the time for a substitute. This one ingredient adds a savory, garlicky, herby flavor that permeates the whole dish. For the other cheeses, I highly recommend buying a block of Gruyère and Parmesan and grating them yourself. Pre-shredded cheeses often contain anti-caking agents that can prevent them from melting as smoothly. And when it comes to mayonnaise, a good quality, full-fat version like Duke’s will lend the best flavor and creamy texture.

Step-by-Step Instructions For Making Spinach Casserole

Making this spinach casserole is wonderfully straightforward, but a few key techniques will ensure you get the perfect creamy texture and rich flavor every single time. We’ll walk through each step together, so you can feel confident in your kitchen. The most important part is the prep work, particularly with the spinach. Get that right, and the rest is a breeze.

Step 1: Preheat Oven and Prepare Your Dish

First things first, let’s get that oven going. You’ll want to preheat it to 375°F (190°C). This moderate temperature is perfect for heating the casserole through and getting the top golden brown without it becoming dry. While the oven preheats, grab a 1.5 or 2-quart baking dish. This could be a classic 8×8 inch square dish or a similar-sized oval gratin dish. Give it a light coating of butter or non-stick cooking spray to prevent sticking and make cleanup easier.

Step 2: Dry the Spinach Thoroughly

I cannot stress this step enough, it is the absolute key to avoiding a watery casserole. Once your frozen spinach is completely thawed, you need to squeeze out as much liquid as humanly possible. I like to do this in small handfuls, squeezing tightly over the sink until no more water drips out. You’ll be shocked at how much liquid comes out. For an even drier result, you can place the thawed spinach in a clean kitchen towel or cheesecloth and wring it out, or press it firmly against a fine-mesh sieve with the back of a spoon.

Step 3: Combine the Creamy Base

In a large mixing bowl, it’s time to build our flavorful, creamy base. Make sure your Boursin cheese and cream cheese are nicely softened to room temperature. This will help them combine smoothly without any lumps. Add the sour cream and mayonnaise to the bowl as well. Using a flexible spatula, stir and press everything together until you have a smooth, homogenous mixture. Take a moment to scrape down the sides of the bowl to make sure everything is incorporated.

Step 4: Add Cheeses, Egg, and Seasonings

Now we’ll add the remaining flavor components. Stir in half a cup of your shredded Gruyère cheese, all of the grated Parmesan, and the lightly beaten egg. The egg acts as a binder, giving the casserole a bit of structure so it’s not too loose. Next, add the seasonings: freshly grated nutmeg, black pepper, and just a pinch of salt. Remember the cheeses are salty, so you don’t need much. Finally, stir in 1/4 cup of the crispy fried onions. This adds a lovely savory flavor and texture right inside the casserole itself.

Step 5: Fold in the Spinach

Add your very dry, squeezed spinach to the cheese mixture. Use your spatula to gently fold it in. Continue folding until the spinach is evenly distributed throughout the creamy base and you don’t see any large clumps of plain cheese mixture or spinach. The color should be a consistent, lovely pale green.

Step 6: Assemble the Casserole

Spoon the spinach mixture into your prepared baking dish. Spread it out into an even layer, reaching all the corners. A smooth, level surface will ensure it bakes evenly and looks beautiful when it comes out of the oven.

Step 7: Add the Topping

This is the grand finale before baking. Sprinkle the remaining half cup of shredded Gruyère cheese evenly over the top of the spinach mixture. Then, cover the entire surface with the remaining one cup of crispy fried onions. Be generous here, as this topping creates the most wonderfully crunchy and savory crust.

Step 8: Bake to Perfection

Place the dish in your preheated oven and bake for 25 to 30 minutes. You’re looking for a few key visual cues. The casserole should be hot and bubbling around the edges, and the crispy onion topping should be a beautiful deep golden brown. The aroma filling your kitchen will be absolutely heavenly.

Step 9: Let it Rest

This might be the hardest step, but it’s important! Once you pull the casserole out of the oven, let it rest on a wire rack for about 5 to 10 minutes before serving. This allows it to set up slightly, making it easier to scoop and serve. It will still be plenty hot, I promise.

How To Serve Spinach Casserole

This incredible spinach casserole is so versatile, it feels right at home in almost any setting, from a formal holiday dinner to a casual weeknight meal. Its creamy texture and rich, savory flavor make it a perfect companion to a wide variety of main courses. It’s the kind of side dish that people get genuinely excited about seeing on the table.

For big celebratory meals, this casserole is a non-negotiable. It’s an absolute star on the Thanksgiving, Christmas, or Easter table. It pairs beautifully with roasted turkey, glazed ham, or a standing rib roast. Its savory, cheesy goodness provides a wonderful contrast to the other traditional side dishes. I love to place it right next to the mashed potatoes and stuffing, as it really rounds out the plate.

But don’t just save it for the holidays! Here are a few more ways to enjoy this delicious dish:

• With Weeknight Roasts: Serve a generous scoop alongside a simple roasted chicken or pork loin. It instantly elevates a simple meal into something special.
• Next to Grilled Meats: It’s a fantastic side for grilled steak, pork chops, or even chicken breasts. The cool, creamy casserole is a lovely balance to the smoky char of the grill.
• As a Star Vegetarian Main: While it’s often a side, this spinach casserole is rich enough to be the main event for a vegetarian meal. Serve it with a fresh green salad and some crusty bread for a complete and satisfying dinner.
• The Ultimate Dip: Surprise! This casserole doubles as an incredible hot dip. Serve it warm with sturdy tortilla chips, pita bread, or an assortment of fresh veggie sticks for a party appetizer that will disappear in minutes.

No matter how you serve it, I recommend presenting it right in the beautiful baking dish it was cooked in. The golden-brown, bubbly top is part of its charm. Just be sure to have a good, sturdy serving spoon ready, because everyone will be eager to dig in for a scoop, or two.

How To Store & Reuse Spinach Casserole Leftovers

If you find yourself with leftovers of this spinach casserole, count yourself lucky. It’s just as delicious the next day, and it reheats beautifully. Proper storage is key to making sure it tastes just as good as when it was freshly baked. You can enjoy it as a simple side dish again or get a little creative and transform it into something new.

First, let the casserole cool down to room temperature completely before storing. Covering it while it’s still warm can create condensation, which can make the topping soggy. Once cooled, you can either cover the baking dish tightly with plastic wrap or aluminum foil, or transfer the leftovers to an airtight container. It will keep well in the refrigerator for up to 4 days.

When you’re ready to enjoy it again, you have a couple of options for reheating. The best method is in the oven, as it helps re-crisp the onion topping. Simply place the desired amount in an oven-safe dish, cover with foil, and bake at 350°F for about 15-20 minutes, or until heated through. Remove the foil for the last 5 minutes to let the top get crunchy again. If you’re in a hurry, the microwave works just fine. Reheat individual portions on a microwave-safe plate in 30-second intervals until hot.

Want to do more than just reheat? Here are some of my favorite ways to reuse leftover spinach casserole:

• Stuffed Chicken Breast: Cut a pocket into a chicken breast, stuff it with a few spoonfuls of the cold casserole, and bake until the chicken is cooked through. It creates a self-saucing, incredibly flavorful main dish.
• Quick Pasta Sauce: Warm up the leftover casserole in a saucepan over low heat, adding a splash of milk or cream to thin it into a luscious pasta sauce. Toss with your favorite cooked pasta, like penne or fettuccine.
• Omelet or Frittata Filling: Spoon some of the casserole into the center of an omelet before folding it over, or mix it into your egg base for a hearty and flavorful frittata.
• Gourmet Grilled Cheese: Spread a thin layer of the cold casserole between two slices of bread with some extra cheese for the most decadent, amazing grilled cheese you’ve ever had.

With so many delicious possibilities, you might find yourself making a double batch of this spinach casserole just to ensure you have plenty of leftovers to play with during the week. It’s truly the side dish that keeps on giving.

Substitutions & Variations For Spinach Casserole

One of the things I love most about a good casserole recipe is how easy it is to adapt to your own tastes or what you happen to have on hand. While I believe this spinach casserole is perfect as written, there’s always room to play in the kitchen. Whether you need to accommodate a dietary preference or just want to try a new flavor combination, here are some tried-and-true substitutions and variations.

Don’t be afraid to experiment with the cheeses. The Boursin is the star, but the other cheeses can be swapped. If you don’t have Gruyère, a good quality sharp white cheddar, Monterey Jack, or even a smoky Fontina would be delicious. For the Parmesan, Pecorino Romano is a great alternative with a similar salty, nutty profile. Just remember to grate your own for the best results.

If you’re looking to change up the flavor profile or add more texture, here are a few ideas to get you started:

• Make it a Spinach Artichoke Casserole: Embrace the classic dip flavor by folding in a can of drained and roughly chopped artichoke hearts along with the spinach. It’s a natural and delicious combination.
• Add a Little Heat: For those who like a spicy kick, add a pinch of red pepper flakes or a few dashes of your favorite hot sauce to the cream cheese mixture. A can of drained, diced green chiles would also be a wonderful addition.
• Change Up the Crunch: If you’re not a fan of crispy fried onions, you can substitute the topping with panko breadcrumbs tossed with a little melted butter, or even crushed Ritz crackers for a classic buttery crunch.
• Introduce Some Protein: To make it a more substantial dish, consider adding some crumbled cooked bacon, diced ham, or shredded chicken to the mixture.

For a gluten-free version of this spinach casserole, the main thing to watch out for is the crispy fried onions. Many store-bought brands contain wheat flour. You can either purchase a certified gluten-free brand or use one of the alternative toppings mentioned above, like cheese or gluten-free breadcrumbs. The rest of the ingredients are naturally gluten-free, but always double-check your labels to be safe. This recipe is wonderfully forgiving, so have fun making it your own.

5 FAQs About Spinach Casserole

Here are some straightforward answers to the most common questions I receive about this spinach casserole recipe.

Why is my spinach casserole watery?

A watery spinach casserole is almost always caused by one thing: excess moisture in the spinach. Frozen spinach holds a tremendous amount of water, and if it’s not removed properly before mixing, that water will release during baking, creating a soupy or separated consistency in your final dish. It’s a very common issue, but luckily, it’s also very easy to prevent.

The solution is to be absolutely relentless when it comes to squeezing the thawed spinach dry. Don’t just give it a gentle pat. You need to wring it out with some real effort. I recommend working in small batches, taking a handful of the thawed spinach and squeezing it between your palms over the sink until not a single drop of water comes out. For an even more effective method, you can place the spinach in the center of a clean, sturdy kitchen towel or a few layers of cheesecloth, gather the ends, and twist and squeeze with all your might. A potato ricer is another fantastic tool for this job. Getting the spinach as dry as a bone is the single most important step for achieving that perfect, creamy, and sliceable casserole texture.

Can I make this spinach casserole ahead of time?

Absolutely! This is one of the reasons this spinach casserole is such a fantastic choice for holidays or entertaining. You can fully assemble the entire casserole a day or two in advance, which is a huge help when you have a lot of other dishes to prepare. Simply follow all the steps right up until baking. Prepare the spinach mixture, spread it in your greased baking dish, and add the cheese topping.

However, I recommend waiting to add the final layer of crispy fried onions until just before you bake it. This will ensure they stay as crunchy as possible. Cover the assembled, untopped casserole tightly with plastic wrap or foil and store it in the refrigerator for up to 48 hours. When you’re ready to bake, remove it from the fridge for about 30 minutes to take the chill off, then sprinkle on the crispy onions and bake as directed. You may need to add about 5 to 10 minutes to the total baking time since you’re starting with a cold dish.

Can I use fresh spinach instead of frozen for this spinach casserole?

You can certainly use fresh spinach, but it requires an extra step and a much larger quantity than you might think. Spinach wilts down significantly when cooked. As a general rule of thumb, you will need about 1.5 to 2 pounds of fresh spinach to equal the amount in two 10-ounce packages of frozen spinach. The best way to prepare it is to sauté it in a large skillet or pot with a tiny bit of olive oil or butter until it is completely wilted. You’ll likely need to do this in batches.

Once the spinach is cooked, let it cool slightly, then you must treat it just like the thawed frozen spinach. It will have released a lot of water during cooking, so you need to squeeze it completely dry using one of the methods described above. After it’s cooked and thoroughly squeezed, you can proceed with the recipe exactly as written. While it’s an extra step, some people prefer the flavor and texture of fresh spinach, so it’s a great option if you have it on hand.

What is the best way to thaw frozen spinach for this casserole?

There are a couple of easy and effective methods for thawing your frozen spinach, and the best one for you just depends on how much time you have. The most hands-off method is to simply place the unopened packages of frozen spinach in a bowl or on a plate in the refrigerator overnight. By the next day, it will be completely thawed and ready for you to squeeze.

If you’re short on time, the microwave is your best friend. Remove the spinach from its packaging and place it in a microwave-safe bowl. Use the defrost setting on your microwave, or microwave it on high in 1-minute intervals, stirring in between, until it’s fully thawed. Be careful not to actually cook the spinach. Once it’s thawed, let it cool down enough so that you can handle it comfortably before you begin the all-important squeezing step. Do not skip the squeezing step, even if you use the microwave.

Is this spinach casserole gluten-free?

This spinach casserole can easily be made gluten-free with one small adjustment. The creamy, cheesy filling of the casserole, including the spinach, cheeses, sour cream, and seasonings, is naturally gluten-free. The ingredient you need to be mindful of is the crispy fried onions used both in the filling and as the topping. Most of the popular, commercially available brands of crispy fried onions are made with wheat flour, which means they contain gluten.

To make the recipe completely gluten-free, you have two options. First, you can seek out a brand of certified gluten-free crispy fried onions, which are becoming more widely available in grocery stores. Alternatively, you can swap the topping for something else entirely. A great gluten-free option would be to use crushed gluten-free crackers or breadcrumbs tossed with melted butter, or simply top it with extra cheese and a sprinkle of smoked paprika for color. The casserole will be just as delicious without the traditional onion topping.

Try These Recipes Next

If you loved the savory, cheesy goodness of this spinach casserole, you’ll definitely enjoy some of my other favorite recipes that feature spinach.

• Tomato Bacon Spinach Quiche: A perfect dish for brunch or a light lunch, combining classic flavors in a flaky pie crust.
• Spinach, Leek and Goat Cheese Tart: An elegant and flavorful tart with tangy goat cheese and sweet leeks that’s sure to impress.
• Leek and Spinach Frittata: A simple, healthy, and satisfying frittata that’s wonderful for any meal of the day.

Give these a try the next time you’re looking for a delicious way to use one of my favorite greens!