How to Make It
In a bowl, stir together 1 cup mozzarella cheese and the pepperoni.
Place each chicken breast, smooth-side-up, on a work surface. Place the palm of one hand on top of one breast, then insert a pairing knife to make a length-wise pocket in each breast. Divide the mozzarella filling between the breasts, stuffing them full. Rub the chicken with the oil, then season all over with 3/4 teaspoon salt and 1/2 teaspoon pepper. Divide the tomato sauce over the tops of the breasts, spreading evenly, then sprinkle with additional mozzarella.
Preheat the oven to 425°F.
Place the chicken breasts on an oiled baking sheet and bake until cooked through, about 30 minutes. Let stand 5 minutes before slicing and serving.