Preheat grill.
Place burgers and onion slices on grill. Season with salt and pepper. Cook about 6 minutes per side or until burgers are cooked thoroughly and the onions are soft and starting to brown.
Place 2 slices of cheese on each burger patty and cook 1 more minute to melt cheese.
Serve burgers on buns with tomato slices and barbecue sauce.
Servings: 4