Lightly shape ground beef into four 3/4-inch thick patties. Brush onion with oil. Place patties in center of grid over medium, ash-covered coals; arrange onion around patties. Grill, uncovered, 13 to 15 minutes to medium (160°F) doneness, until patties are not pink in center and juices show no pink color and onions are tender, turning occasionally. Season with salt and pepper after turning. About 1 minute before patties are done, top each with 2 cheese slices.
Combine mayonnaise and mustard; spread on top halves of rolls. Serve burgers and onion in rolls with lettuce and tomato.
Makes 4 servings.
Nutrition information per serving: 845 calories; 55 g protein; 36 g carbohydrate; 52 g fat; 724 mg sodium; 173 mg cholesterol; 9.1 mg niacin; 0.6 mg vitamin B6; 4.4 mcg vitamin B12; 5.3 mg iron; 10.8 mg zinc.
Courtesy of the Cattlemen's Beef Board and the National Cattlemen's Beef Association.