Combine bread crumbs, Parmesan cheese, salt, and black pepper in a shallow bowl; place flour into a second bowl. Whisk egg and lemon juice together in a third bowl.
Dip each chicken breast in egg mixture and dredge in flour; dip again in egg mixture and dredge chicken in crumb mixture until coated. Place coated chicken breasts into a baking dish.
Bake in the preheated oven until chicken is heated through and crumbs are golden, about 20 minutes.
Spoon 2 tablespoons pizza sauce onto each chicken breast, sprinkle each with half the mozzarella cheese, and top each with half the pepperoni slices.
Bake until chicken is no longer pink inside and the juices run clear, about 10 more minutes. An instant-read meat thermometer inserted into the thickest part of a breast should read at least 160 degrees F (70 degrees C).