Trim fat from chops. Combine onion, 3 tablespoons mint, garlic, and pepper in a large heavy-duty, zip-top plastic bag. Add chops; seal bag, and shake until chops are well coated. Marinate chops in refrigerator at least 3 hours, turning bag occasionally.
Remove chops from marinade; discard marinade. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place chops on rack; grill, covered, 5 minutes on each side or to desired degree of doneness.
To serve, place a chop on each individual serving plate. Sprinkle evenly with remaining 2 tablespoons mint.