Prepare the Mushroom Filling: Melt butter in a skillet over medium heat. Add the sliced mushrooms and sauté until golden and softened, about 5-7 minutes. Stir in the garlic, parsley, salt, and pepper. Cook for another 2 minutes. Remove from heat and set aside.

Prepare the Chicken: Preheat your oven to 375°F (190°C). Butterfly each chicken breast by slicing it horizontally without cutting all the way through, creating a pocket. Season the inside and outside of the chicken with salt, pepper, onion powder, and dried parsley. Stuff each chicken breast with 2 slices of mozzarella cheese and an even amount of the mushroom filling. Secure the opening with toothpicks.

Cook the Chicken: Heat a skillet over medium heat with a drizzle of olive oil. Sear the stuffed chicken breasts on each side until golden brown, about 3-4 minutes per side. Transfer the skillet to the oven (or place the chicken in an oven-safe dish) and bake for 15-20 minutes, or until the chicken is cooked through (internal temperature reaches 165°F or 75°C).

Make the Garlic Parmesan Cream Sauce: While the chicken bakes, use the same skillet to make the sauce. Heat olive oil over medium heat, then add minced garlic and sauté for 1 minute until fragrant. Stir in the Dijon mustard, half and half, parmesan cheese, salt, and pepper. Simmer gently for 5 minutes until the cheese melts and the sauce thickens slightly. If desired, add the cornstarch slurry and cook for an additional 2 minutes to thicken further. Stir in the fresh parsley before removing from heat.

Assemble and Serve: Remove the toothpicks from the chicken breasts and pour the garlic parmesan cream sauce over the top. Garnish with additional parsley, if desired. Serve warm with a side of steamed vegetables, mashed potatoes, or crusty bread to soak up the delicious sauce.