1 large sweet onion, cut crosswise into 1/2-inch thick slices
Salt and pepper
4 hamburger buns, split, toasted
Lightly shape Ground Beef into four 3/4-inch thick patties. Brush onion slices with oil.
Place patties and onion slices on grid over medium, ash-covered coals. Grill, covered, 11 to 15 minutes (over medium heat on preheated gas grill, 13 to 14 minutes) until instant-read thermometer inserted horizontally into center registers 160F, turning occasionally. Grill onions, 10 to 12 minutes or until tender, brushing with oil and turning occasionally.
Season burgers and onions with salt and pepper as desired.
Line bottom of each bun with lettuce. Place burgers on top of lettuce. Top with tomatoes and grilled onions. Close bun.