See "The Great American Hamburger" recipe for main ingredients.
6 slices sharp cheddar cheese (1/4 inch thick), Swiss cheese or 6 pieces Roquefort
Follow the instructions through cooking the first side of the burgers in step 4. After you flip them, place a piece of cheese on top. Cover the grill and cook the burgers until the cheese is melted and the meat is done to taste. Check after 3 minutes.