MIX dressing, 2 Tbsp. steak sauce, sugar and garlic until blended. Pour over steak in shallow glass dish; turn steak to coat both sides. Refrigerate 1 hour to marinate.
HEAT oil in large skillet on medium-high heat. Add onions; cook and stir 5 min. Cover; cook on medium-low heat 15 min. or until onions are golden brown, stirring occasionally.
MEANWHILE, heat broiler. Remove steak from marinade; discard marinade. Place steak on broiler pan. Broil steak, 4 inches from heat, 8 min. on each side or until done (160ºF). Remove from heat; cover. Let stand 5 min. before cutting across the grain into thin slices. Stir remaining steak sauce into onions. Serve with meat.