Spray a 9x13 oven-safe dish or 9x13 foil tray with non-stick cooking spray.
Slice chicken in half horizontally through the thickness of the breast. Pound to ½ an inch thick. You can make this easier by placing chicken between sheets of plastic wrap or waxed paper when pounding.
Sprinkle the chicken with salt and pepper. In the middle of the chicken, place four slices of pepperoni and about 1 Tbsp. of shredded mozzarella. Roll the chicken, tucking in the edges, and place in the baking dish or foil tray.
Pour pizza sauce over each chicken breast. Top with remaining mozzarella and pepperoni slices.
Place two layers of aluminum foil over the tray and seal the edges well.
Label and freeze.
Take the tray out of the freezer. Thaw.
Preheat oven to 375°.
Place the tray in the oven. Bake for 30 to 40 minutes (depending on the thickness of the chicken breast) or until a meat thermometer reads 165°.