1. Preheat the oven to 400°F (200°C).

2. Melt the butter in a large oven-safe skillet over medium heat and cook the garlic for about 1 minute, until fragrant.

3. Add the mushrooms, parsley, salt, and pepper, and cook, stirring occasionally, until the mushrooms are soft, then remove from heat and let cool slightly.

4. Pat the chicken breasts dry and season both sides with salt, black pepper, onion powder, and dried parsley.

5. Cut a deep pocket into the thickest side of each chicken breast with a sharp knife, without cutting all the way through.

6. Place 2 slices of mozzarella inside each chicken pocket, then divide the mushroom mixture evenly among the breasts and stuff it into the pockets.

7. Sprinkle about 1 tablespoon of Parmesan cheese into each pocket and secure with toothpicks if needed.

8. Return the skillet with the remaining garlic butter juices to medium-high heat and sear the stuffed chicken for 3–4 minutes per side, until golden brown.

9. Cover the skillet and transfer it to the oven, baking for about 20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).

10. For the optional sauce, transfer the cooked chicken to a plate, leaving the pan juices in the skillet.

11. Heat the olive oil in the same skillet and sauté the garlic for about 1 minute, then stir in the Dijon mustard and half-and-half and bring to a gentle simmer.

12. Add any remaining mushrooms and the grated Parmesan cheese, stirring until melted, then stir in the cornstarch slurry if a thicker sauce is desired.

13. Season with salt and pepper, stir in the fresh parsley, then return the chicken to the skillet and spoon the sauce over the top before serving.