HEAT barbecue to medium heat. Meanwhile, heat oil in large nonstick skillet on medium-high heat. Add onions; cook 5 min., stirring frequently. Cover with lid. Simmer 20 min. or until onions are golden brown, stirring frequently.
MEANWHILE, grill patties 5 min. on each side or until done (160ºF). Stir barbecue sauce into onion mixture.
FILL buns with tomatoes, burgers, Singles and onion mixture.