HEAT grill to medium-high heat. Combine meat and 2 Tbsp. barbecue sauce; shape into 4 patties. Spray both sides of onion slices with cooking spray; set aside.
GRILL patties 4 min. on each side or until done (160ºF), adding onions to the barbecue after 2 min. and cooking until crisp-tender, turning occasionally. Cover burgers with Singles; grill until melted. Top with remaining barbecue sauce.
FILL buns with burgers, onions and tomatoes.