In medium bowl, combine ground beef, onion and 2 tablespoons barbecue sauce, mixing thoroughly. Shape into four 1/2-inch-thick patties. Place patties on grid over medium ash-covered coals. Grill 14 to 16 minutes or until centers are no longer pink, turning once. Approximately 1 minute before end of cooking time, brush top side of burgers with additional barbecue sauce; top each with 1 slice cheese. Cook until cheese slices are melted. Line bottom half of each roll with lettuce and tomato; top with cheeseburger. Close sandwiches.