Slice the block of mozzarella cheese into even slices. Using a small sharp knife cut a pocket into the front of each chicken breast. Be careful not to cut all the way through the chicken breast or the filling will come out. Stuff each chicken breast with 2 slices of cheese, and 6-8 pepperoni slices.
Place flour into a shallow dish or pie plate. In another shallow dish beat eggs, and in another shallow dish add the bread crumbs. Coat each chicken breast first in the flour mixture, then in the egg mixture, and finally in the breadcrumb mixture. During the coating process make sure to press down on the pocket to keep the filling from falling out.
Pre-heat the oven to 350 degrees and line a large baking sheet with aluminum foil. Spray the aluminum foil lightly with cooking spray. Heat vegetable oil in a large skillet over medium-high heat. Test the oil to make sure it’s hot enough by dropping a small pinch of breadcrumbs into the oil. The bread crumbs should sizzle a lot if it’s hot enough. Once the oil is hot enough, place chicken breasts into the oil and fry until golden brown and crispy on each side (about 2-3 minutes). Remove the chicken from the skillet and place onto the baking sheet.
Place chicken into the oven and bake for 25 minutes or until the chicken is fully cooked (has reached and the internal temperature of 165 degrees).