Preheat oven to 400°F.

Make the Mushroom Filling:
In a large oven-safe skillet, melt butter over medium heat. Add mushrooms, garlic, salt, pepper, and parsley. Cook until mushrooms are tender and most of the liquid has evaporated. Remove from heat and divide into four portions.

Prepare the Chicken:
Pat chicken breasts dry. Season both sides with salt, pepper, onion powder, and dried parsley. Slice a pocket into the side of each breast.

Stuff the Chicken:
Place 2 slices of mozzarella inside each pocket, followed by one portion of the cooked mushrooms. Sprinkle about 1 tablespoon of Parmesan cheese on top of the filling. Use toothpicks to secure the opening.

Sear and Bake:
Reheat the same skillet over medium-high heat. Sear each stuffed breast until golden brown on both sides. Cover the skillet and transfer to the oven. Bake for 20–22 minutes, or until the chicken is cooked through (internal temperature reaches 165°F). Transfer chicken to a plate.

Make the Sauce:
On low heat, add olive oil and garlic to the same skillet. Sauté for about 1 minute. Stir in Dijon mustard and half and half. Simmer gently. Add remaining mushrooms (if any), Parmesan cheese, salt, pepper, and parsley. If the sauce is too thin, mix in the cornstarch slurry to thicken.

Serve:
Return the stuffed chicken breasts to the skillet and spoon sauce over the top. Serve warm.