Pre-heat oven to 350°F. Spray a baking dish with cooking spray.
In a large bowl, add all Cheese stuffing ingredients and stir until well combined. Set aside.
On the side of a chicken breast cut a slit to create a "pocket." Do not slice all the way through.
Stuff each chicken breast with 1/4 of the cheese stuffing. You do not need to seal the chicken.
In a large skillet, heat oil until it shimmers.
While the oil heats, place flour, salt, pepper, and garlic powder in a shallow dish.
Press each side of the stuffed chicken breasts in flour, shaking off excess.
Cook the chicken breasts in the hot oil until the bottom is golden brown, turn and cook the other side until it is golden brown. (Does not have to cook all the way through. We just want to brown the chicken.)
Remove the stuffed chicken breasts from pan and place in a baking dish.
Bake the stuffed chicken breasts for about 20 minutes, or until juices run clean and the temperature of the center of chicken reaches 165°F.
Add butter to the same pan that you cooked the chicken. Add onions and cook for about 2 minutes.
Add mushrooms and cook for about 5 minutes or until the onions are translucent.
Add the Holland House® Marsala Cooking Wine to the pan and simmer on low heat.
Add the heavy cream and cook until the sauce reduces by half.
Place the cooked stuffed chicken breasts on a serving dish and top each with the cooked onions, mushrooms, and Marsala sauce.