1. Pan fry Caprese chicken over medium heat until cooked through, about 5-7 minutes on each side.
2. Slice bread open and lightly butter; broil until lightly browned.
3. Heat marinara sauce in a saucepan or microwave until heated through.
4. Remove bread; place chicken on bread and top immediately with mozzarella cheese (place back under broiler until cheese melts if desired).
5. Drizzle with marinara sauce.