Prepare the Chicken

• Start by preheating the oven to 375°F (190°C). Season the chicken breasts with salt and pepper on both sides.
• In a large skillet, heat 2 tablespoons of olive oil over medium-high heat. Once the oil is hot, add the chicken breasts and sear them for 3-4 minutes on each side until golden brown. The chicken does not need to be fully cooked through at this point, as it will finish cooking in the casserole.
• Remove the chicken from the skillet and set it aside. In the same skillet, add the butter and allow it to melt. Sauté the sliced onions, carrots, and mushrooms for about 5-7 minutes, or until they begin to soften. Add the minced garlic and cook for another minute until fragrant.

Make the Sauce

• In the same skillet with the sautéed vegetables, add the white wine and use a wooden spoon to scrape up any browned bits from the bottom of the pan. Let the wine simmer for 2-3 minutes to reduce slightly.
• Stir in the chicken broth, heavy cream, Dijon mustard, and fresh thyme. Bring the sauce to a simmer and let it cook for 5-7 minutes, allowing it to thicken slightly. Season the sauce with salt, pepper, and lemon juice to taste.

Assemble the Casserole

• In a large baking dish (9×13-inch works well), place the seared chicken breasts at the bottom. Pour the vegetable and sauce mixture over the chicken, making sure the chicken is evenly covered.
• Sprinkle the shredded Gruyère cheese evenly over the top of the casserole. If you like a bit of crunch, you can also sprinkle the top with panko breadcrumbs before adding the cheese.
• Place the casserole in the preheated oven and bake for 30-35 minutes, or until the chicken is fully cooked through (reaching an internal temperature of 165°F or 75°C) and the sauce is bubbling.

Serve and Enjoy

• Once the casserole is finished baking, remove it from the oven and let it rest for 5 minutes before serving. Garnish with freshly chopped parsley for added color and freshness.
• Serve the French Chicken Casserole hot, alongside your favorite side dishes. It pairs wonderfully with a simple green salad, rice, or roasted vegetables.