Preheat oven to 400F.
Melt 4 tablespoons of butter in a large oven-proof skillet (if you have one--if not, use a regular skillet) over medium heat. Add 4 minced garlic cloves and sauté just until fragrant, 30-60 seconds. Add the mushrooms and parsley. Cook over medium heat, stirring occasionally, until the mushrooms are soft.
Season each chicken breast on both sides with the onion powder, salt, and pepper.
Butterfly each chicken breast to create a large "pocket" in the center. Stuff each chicken breast with two slices of mozzarella cheese, the cooked mushrooms, and Parmesan cheese. Use toothpicks to close each opening of the pockets.
Heat the oven-proof skillet over medium heat again. Add the olive oil and once hot, place each stuffed chicken breast into the skillet. Cook the chicken breasts on each side until golden brown.
Cover the oven-proof skillet with an oven-proof lid or foil. Place the whole skillet in the oven and bake at 400F for 20 minutes, or until the chicken reaches an internal temperature of 165F.
Note: if you do not have an oven-safe skillet, bake the chicken in a glass baking dish instead.
While the chicken is baking, heat 4 tablespoons butter in a large skillet over medium heat. Add 2 minced garlic cloves and sauté until fragrant, 30-60 seconds. Add the dijon mustard and 1 1/2 cups of the half and half; whisk until smooth.
Mix the cornstarch and water together, then whisk into the sauce. Mix in the Parmesan cheese. Bring the sauce to a simmer, stirring often. Reduce the heat to low and add salt and pepper to taste along with the chopped fresh parsley. If needed, add the remaining 1/2 cup of half and half to thin the sauce if it becomes too thick.
Remove the toothpicks from the chicken breasts, then nestle the chicken into the cream sauce. Serve warm.