For the filling, melt butter in a pan over medium heat. Add mushrooms and garlic, cooking until mushrooms are golden. Remove from heat, stir in cream cheese, Parmesan, parsley, salt, and pepper.
Cut a pocket into each chicken breast. Stuff with mushroom mixture and secure with toothpicks.
Season chicken with salt, pepper, and coat with breadcrumbs.
Heat olive oil in a skillet over medium-high heat. Sear chicken on both sides until golden.
Transfer to oven and bake for 20-25 minutes, or until chicken is cooked through.
Notes
Ensure chicken is cooked through to an internal temperature of 165°F (75°C).