Creamy mushroom and chicken bake features tender chicken smothered in a rich mushroom cream sauce. This high protein, family-friendly chicken casserole is elegant comfort food that's easy enough for weeknights.
Preheat oven to 400°F and lightly grease a large baking dish.
Heat 2 tbsp butter in a skillet over medium-high heat. Sauté sliced mushrooms and onions for 6–7 minutes until softened and moisture evaporates.
Add minced garlic and dried thyme; cook for 1 minute, stirring constantly, then transfer mixture to a bowl.
Add remaining butter to the skillet. Season chicken breasts with salt and pepper, then sear chicken on both sides for 2–3 minutes until lightly golden.
Arrange seared chicken in baking dish. Top with sautéed mushroom mixture, pour over heavy cream, and sprinkle mozzarella cheese evenly.
Bake uncovered for 22–26 minutes until chicken reaches 165°F internal and cheese is bubbling. Let rest for 5 minutes, garnish with chopped fresh parsley, and serve hot.