Combine cola, oil and steak seasoning in a large shallow pan. Add steaks and onion. Marinate in refrigerator, at least 30 minutes. Remove steaks and onion from marinade; discard marinade.
Grill steaks and onions 16 to 20 minutes, turning once, until steaks reach desired doneness and onions are tender and charred. Remove from grill and let steak rest 5 minutes before serving.