French Onion Chicken Casserole

Rich, cheesy, and loaded with onion flavor, this delicious French Onion Chicken Casserole is perfect for weeknight dinners.

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

For such a decadent casserole, the ingredient list isn't especially long or fussy. Here's what you'll need:

• Yellow onions: Make sure to slice them thinly and evenly so they caramelize 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: Add sliced onions to a heated skillet with a splash of oil. Pop a lid on the skillet, and let them cook until they're soft and brown. Stir occasionally, but give them time to begin to soften and caramelize without much agitation.
• 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 butter has melted. Add the flour over the onion mixture. Cook 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.
• Step 3. Add peas, and 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. 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.

Test Kitchen Tips for This Chicken Casserole

The Test Kitchen professionals that developed and tested this recipe have a few pointers that will help you make it, too:

• 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.
• You can cook the onions ahead, then chill them in the refrigerator to get ahead. Just briefly reheat them in a skillet with oil before adding the flour and continuing with the recipe.
• Feel free to add other cheeses. Mild mozzarella and Gruyere are good partners to the delicately sweet onions, but you could really pump up the flavor with Colby Jack, even Cheddar.
• Add other veggies if you want. When you stir in the frozen peas, you could also add corn. For vegetables that take a little more time to cook (like bell peppers and carrots), stir them in with the onions earlier, or cook them separately and add when you add the peas.
  

What To Serve With French Onion Chicken Casserole

Since this casserole is so rich, 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 Store and Freeze

One of the best things about most chicken casseroles 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.

For any leftovers, store them in an airtight container in the fridge up to 4 days.

Editorial contributions by Alyssa Sybertz.