Make the stuffing: Melt butter in a large skillet over medium heat.
Add minced garlic and cook for 30 seconds until fragrant.
Add chopped cremini mushrooms and cook 10-15 minutes until softened and browned.
Stir in Parmesan cheese, parsley, salt, and pepper; set aside to cool.
Prepare the chicken: Pound chicken breasts to even thickness.
Cut a slit lengthwise in each breast to create a pocket.
Fill pockets with mushroom stuffing and secure with toothpicks.
Cook the chicken: Melt butter in a large skillet over medium-high heat.
Season stuffed chicken breasts with salt and pepper.
Cook chicken breasts 5-7 minutes per side until cooked through.
Sprinkle cooked chicken breasts with Parmesan cheese.
Serve immediately with desired sides.