Preheat oven to 375F and spray a 13 x 9 inch casserole pan with cooking spray.
Heat 1 tbsp. butter and 1/2 tbsp. oil in a large skillet over medium high heat.
Add mushrooms and onions and saute until tender, about 5 minutes.
Add minced garlic and saute for another 30 seconds.
Pour half of this mixture into the prepared casserole pan and set the other half aside.
Season chicken with paprika, garlic powder, and Italian seasoning and heat the remaining butter and oil in the skillet.
Add chicken to the skillet and cook 2 minutes on each side.
Transfer chicken breasts to the casserole pan on top of the mushrooms and onions.
Add chicken stock to the skillet and scrape the bottom of the pan to bring the browned bits of flavor out.
Bring to a boil and let cook until chicken stock has reduced, about 5 minutes.
Add the remaining mushroom/onion mixture on top of the chicken breasts, sprinkle with mozzarella cheese, and pour chicken stock sauce over the top.
Bake for 15 minutes and serve.
ENJOY!