Lightly shape Ground Beef into four 3/4-inch thick patties. Brush onion with cooking spray.
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 160°F, turning occasionally. Grill onions, 10 to 12 minutes or until tender, brushing with cooking spray 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.
Nutrition information per serving: 400 Calories; 124.2 kcal Calories from fat; 13.8 g Total Fat; 5.3 g Saturated Fat; 0.2 g Trans Fat; 1.3 g Polyunsaturated Fat; 5.3 g Monounsaturated Fat; 126 mg Cholesterol; 327 mg Sodium; 25.6 g Total Carbohydrate; 0.6 g Dietary Fiber; 4.8 g Total Sugars; 43.1 g Protein; 0 g Added Sugars; 88.5 mg Calcium; 5.8 mg Iron; 665 mg Potassium; 0 mcg Vitamin D; 0.4 mg Riboflavin; 11.2 mg NE Niacin; 0.7 mg Vitamin B6; 4.3 mcg Vitamin B12; 386 mg Phosphorus; 10 mg Zinc; 43.5 mcg Selenium; 149.6 mg Choline;