Preheat oven to 400°F.
Butterfly the chicken breasts and season with salt and pepper, inside and out.
Open each chicken breast, and add 1 teaspoon of minced garlic to each one. Spread it evenly.
Add about 5-6 pepperoni slices to each butterflied chicken breast on top of the garlic.
Add about 1/4 cup of shredded mozzarella cheese to each chicken breast.
Close the chicken over tightly, and transfer the chicken breasts to a lightly greased baking dish.
Pour the marinara over top of the chicken and into the dish.
Sprinkle the remaining mozzarella cheese, followed by the Italian seasoning, over the chicken.
Top the dish with the remaining pepperoni slices.
Bake uncovered for 20-25 minutes or until chicken is tender and fully cooked.
You can also broil the top layer, for the last 5 minutes of baking. Watch it closely to not burn it.