PREHEAT grill to medium-high heat. Combine 1 lb. extra lean ground beef and 2 Tbsp. Bull's Eye Barbecue Sauce; shape into 4 (1/2-inch-thick) patties. Spray both sides of 4 red onion slices with cooking spray; set aside.
GRILL patties 4 min. on each side or until cooked through, adding onions to the grill after 2 min. and cooking until crisp-tender, turning occasionally. Top each burger with a Singles Light Slice; continue grilling until Singles are melted.
PLACE burgers on bottom halves of 4 split whole wheat hamburger buns; top evenly with additional 2 Tbsp. barbecue sauce. Cover each burger with 1 onion slice, 1 tomato slice and top half of bun.