Preheat oven to 375°F (190°C).
Butterfly chicken breasts and season with salt and pepper.
Sauté mushrooms in 2 tbsp butter until golden brown.
Mix mushrooms with mozzarella cheese.
Stuff chicken breasts with mushroom-cheese mixture.
Melt remaining butter with minced garlic in the skillet.
Cook stuffed chicken in garlic butter for 5 minutes per side.
Transfer skillet to oven and bake for 15-20 minutes.
Check internal temperature reaches 165°F (74°C).
Garnish with parsley and serve your cheesy mushroom chicken.