Combine onion, vinegar, oil, garlic, salt, and pepper in a large resealable plastic bag. Add lamb chops; coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 2 hours.

Preheat an outdoor grill for medium-high heat.

Remove lamb from marinade and leave any onions on that are sticking to the chops; discard marinade. Wrap exposed ends of bones with aluminum foil to keep them from burning.

Cook lamb chops on the preheated grill until grill marks appear and to desired doneness, about 3 minutes per side for medium. An instant-read thermometer inserted into the center should read at least 130°F (54°C).