Cheeseburgers with Grilled Onion

1 1/2 pounds ground beef
4 (1/2 inch thick) slices sweet onion
Vegetable oil
8 (1 ounce) slices Swiss cheese
1/4 cup mayonnaise
1 tablespoon Dijon-style mustard
4 crusty rolls, split, toasted
Romaine lettuce leaves
Tomato slices

Lightly shape ground beef into four 3/4-inch-thick patties.

Brush onion slices with oil.

Place patties in center of grid over medium, ash-covered coals; arrange onion slices around patties. Grill, uncovered, 13 to 15 minutes to medium (160 degrees F) doneness, or until centers are no longer pink and juices show no pink color.

Grill onions until tender, turning occasionally. Season patties with salt and pepper after turning, as desired.

About 1 minute before patties are done, top each with two cheese slices.

Combine mayonnaise and mustard; spread on top halves of rolls.

Serve burgers and onion slices in rolls with lettuce and tomato.

Makes 4 servings.