French Onion Chicken Casserole

Rich, cheesy, and loaded with onion flavor, this delicious French Onion Chicken Casserole is perfect for chilly nights. Here, we're sharing what makes this stick-to-your-bones casserole so special, plus what you need to make it and how to freeze it so you can make it ahead of time.

What Is French Onion Chicken Casserole?

Like every good chicken casserole, French Onion Chicken Casserole features chicken and cheese enveloped in a decadently creamy base. To give this one-pan dinner true French onion flavor, it features onions two ways: We quickly caramelized fresh onions for the filling and then topped everything with crispy fried onions before baking.

There's also a mixture of shredded mozzarella and Gruyère cheese on top of the casserole to reinforce the delicious French onion-ness of it all.

French Onion Chicken Casserole Ingredients

Yellow Onions
Make sure to slice them thinly and evenly so they brown quickly without burning.

Butter, garlic, flour, and broth
These ingredients round out the onion half of the casserole mixture.

Frozen peas
This casserole is rich and heavy, so we added peas for a pop of green and freshness.

Cooked chicken
You can use rotisserie chicken, leftover roast chicken, or grilled chicken breast from the deli. Or if you'd prefer, roast a few chicken breasts yourself, then chop them into bite-size pieces.

Sour cream, mayonnaise and parsley
The finishers for the chicken half of the mixture, sour cream and mayonnaise, provide tanginess and body while fresh parsley adds brightness. You can also garnish with more parsley just before serving.

Fried onions
Store bought crispy fried onions add some crunch to this rich and creamy dish.

Mozzarella and Gruyere
Top your casserole with these mouthwatering melty cheeses just before baking.

How to Make French Onion Chicken Casserole

Other than sautéing the onions, which admittedly takes some time, this casserole comes together quickly. Here's how:

Step 1. Cook the onions

While you've got oil in a skillet warming up for the onions, go ahead and ready the oven. Preheat to 350°F, and coat an 11- x 7-inch baking dish with cooking spray. If you have a 2 1/2-quart round or oval dish, that will work, too.

Then, with oil heated in your skillet, go ahead and add the onions. Pop a lid on the skillet, and let them cook for 10 to 12 minutes, until softening and browning. Stir occasionally, but give them time to begin to soften and caramelize without much agitation.

After about 12 minutes, remove the lid, and add the salt and pepper. Stir, and put the lid back on. Cook a bit more, about 8 to 10 minutes, until the onions are golden brown and very soft.

The exact time you'll need to get the darker sautéed onions depends largely on how thick the the onion slices are. Keeping them uniform in size will help everything cook down nicely. Use a mandoline for precise sizes if your knife skills lack something to be desired.

Step 2. Add butter, garlic, and flour

When the onions are at the color you want, uncover the skillet, and add the butter and garlic. Cook about 1 minute, until the onion is fragrant and butter has melted.

Next, add the flour over the onion mixture. You need to cook it about 1 to 2 minutes to get the flour nicely cooked and incorporated into the onions. Then, stir in the broth and bring to a simmer. The simmer time will let the flour thicken up nicely with the broth to create a silky sauce.

Step 3. Add peas, stir together other ingredients

Once the sauce is thickened, remove the skillet from the heat, and add in the peas. Then let the mixture sit for 5 minutes while you combine the chicken, sour cream, mayonnaise, and parsley in a separate bowl.

Step 4. Combine and add to pan

Once the onion mixture has cooled a bit, add it to the large bowl with the chicken mixture, and stir. When well combined, transfer the casserole mixture to the baking dish you prepared back in step 1. Spread evenly.

Finish off the casserole with the fried onions and both cheeses—mozzarella and Gruyère.

Step 5. Bake

With all the casserole toppings in place, bake in the preheated oven until the cheese is melted and just golden. This should take about 25 to 30 minutes. Once it's out of the oven, the casserole should cool for about 5 minutes. Then, serve and enjoy.

How to Serve French Onion Chicken Casserole

Since this casserole is so heavy, we like to serve it with a light, fresh green salad alongside for balance. You could also serve it over rice or pasta to soak up the indulgent and creamy sauce.

For a fun family meal or if you're entertaining, you can serve the French Onion Chicken Casserole with crusty French bread for dipping and scooping—no forks necessary.

How to Freeze

One of the best things about chicken casserole is that it can be made ahead of time for service in a pinch, and this French Onion Chicken Casserole is no exception. Since it includes cooked chicken you can assemble the casserole and freeze it before baking, though we recommend waiting to add the shredded cheese and crispy fried onions until right before it goes in the oven so they don't lose their flavor and texture (respectively).

When you're ready to serve, thaw the casserole completely in the fridge overnight, and then proceed with the baking instructions in the recipe. You could bake it from frozen, but you'll need to add some time and periodically check the center of the casserole with a meat thermometer to ensure it's the proper temperature for eating.

Editorial contributions by Alyssa Sybertz.