PREHEAT grill 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 medium doneness (160ºF). Stir steak sauce into onion mixture.
PLACE 1 tomato slice and burger on bottom half of each bun; cover with 1 Singles, 3 Tbsp. of the onion mixture and top of bun.