Preheat your oven to 375°F (190°C).

In a skillet, melt the butter over medium heat.

Add the diced mushrooms and minced garlic; sauté until the mushrooms are tender, about 5-7 minutes.

Remove from heat and stir in the mozzarella cheese, salt, and pepper until combined.

Create a pocket in each chicken breast by slicing it horizontally, being careful not to cut all the way through.

Stuff each chicken breast with the mushroom mixture, pressing down gently to secure the filling.

Season the outside of the chicken with salt and pepper.

Place the stuffed chicken in a baking dish and bake for about 25-30 minutes, until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).

Garnish with freshly chopped parsley before serving.