Preheat Oven: Preheat your oven to 375°F (190°C). Grease a baking dish with cooking spray or butter.

Prepare Chicken: Place each chicken breast on a cutting board. Use a sharp knife to slice a pocket into the side of each chicken breast, being careful not to cut all the way through. Season the inside of each pocket with salt and pepper.

Cook Mushrooms: In a skillet or frying pan, melt the butter over medium heat. Add the minced garlic and sliced mushrooms to the pan. Cook, stirring occasionally, until the mushrooms are tender and golden brown, about 5-7 minutes. Season with salt and pepper to taste.

Stuff Chicken: Spoon the cooked mushroom mixture into the pockets of the chicken breasts, dividing it evenly among them. Sprinkle shredded mozzarella cheese over the mushroom mixture in each pocket.

Secure Chicken: If desired, use cooking twine to tie the chicken breasts closed to secure the filling inside.

Bake: Place the stuffed chicken breasts in the prepared baking dish. Sprinkle grated Parmesan cheese over the top of each chicken breast. Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and the cheese is melted and bubbly.

Garnish and Serve: Remove the stuffed chicken breasts from the oven and let them rest for a few minutes. Garnish with chopped fresh parsley before serving. Serve hot and enjoy the delicious flavors of Cheesy Garlic Butter Mushroom Stuffed Chicken!