Saute onion in oil until it is just browned.
Place onion in a bowl, then add ground beef, garlic, salt and pepper.
Shape into 6 patties.
Put patties in hinged wire grill and place over a solid bed of glowing coals.
Cook about 6 minutes.
Turn burgers and cook second side for 4 minutes.
Place hamburger rolls cut-side down on the barbecue grill to toast.
Top each patty with grated Gruyere cheese and cook for an additional 2 minutes.