Preheat your oven to 375 degrees. Spray a 9 by 13 inch glass baking dish with cooking spray and pour a enough pizza sauce in the bottom to completely cover it. Set dish aside.
Cut each of the four chicken breasts in half, not going all the way through, butterflying them. (See photo above.)
Sprinkle each with salt and pepper. Put 1 to 2 TBSPs of pizza sauce on top of each piece of chicken. Top with 1/4 cup of mozzarella cheese each. Then top each one with your favorite pizza toppings.
Fold the chicken back over and secure closed with two toothpicks.
Place the four stuffed chicken breasts in the prepared pan. Top each piece of chicken with a little more sauce and 1/4 cup mozzarella cheese. Cover the pan with aluminum foil and bake for 40 minutes or until the chicken is completely cooked through.
Remove the chicken from the oven, transfer to plates, and remove toothpicks.