Trim fat from chops; place chops in a heavy-duty, zip-top plastic bag. Add onion, teriyaki sauce, and garlic to bag. Seal bag, and marinate in refrigerator at least 8 hours, turning bag occasionally.
Remove chops from marinade, reserving marinade. Place marinade in a small saucepan; bring to a boil. Remove from heat, and set aside. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place chops on rack; grill, covered, 8 minutes on each side or to desired degree of doneness, basting occasionally with marinade.