1. Preheat oven to 400°F (200°C).
2. Melt butter in a large ovenproof skillet over medium heat. Add minced garlic and sauté until fragrant, about 1-2 minutes.
3. Add sliced mushrooms, salt, pepper, and fresh parsley. Cook until mushrooms are tender and browned. Remove from heat and let cool slightly.
4. Pat chicken breasts dry and season with salt, pepper, onion powder, and dried parsley.
5. Slice each chicken breast horizontally to create a pocket, careful not to cut through.
6. Stuff each pocket with mozzarella cheese and the mushroom mixture. Sprinkle parmesan cheese over the filling.
7. Seal pockets with toothpicks.
8. In the same ovenproof skillet, sear stuffed chicken over medium-high heat in leftover mushroom juices until golden brown on both sides, about 5-7 minutes per side.
9. Transfer skillet to oven and bake uncovered for 20-25 minutes, or until chicken reaches an internal temperature of 165°F and cheese is melted and bubbly.
10. For the optional sauce, sauté minced garlic in the pan drippings with olive oil. Whisk in Dijon mustard and half and half. Simmer gently.
11. Add parmesan cheese and mushroom mixture back into the sauce. Thicken with cornstarch slurry if desired. Season with salt, pepper, and fresh parsley.
12. Return chicken to the sauce to coat before serving.