1-1/2 lb ground beef
4 slices (1/2") sweet onion
8 slices Swiss process cheese food
4 crusty rolls, split, toasted
4 romaine lettuce leaves
4 slices (1/4") tomato
1/4 cup mayonnaise
1 Tbsp Dijon-style mustard
1. Combine sauce ingredients; set aside.
2. Lightly shape ground Beef into four 3/4"-thick patties. Lightly brush onion with oil. Place patties and onion on grid over medium, ash-covered coals. Grill, uncovered, 13 to 15 minutes or until centers of patties are no longer pink and onions are tender, turning occasionally. Season burgers with salt and pepper if desired, after turning. Approx. 1 minute before burgers are done, top each with 2 cheese slices.
3. Meanwhile spread sauce on top half of each roll. Line bottoms of rolls with lettuce and tomato; top with cheeseburger and onion. Close sandwiches.