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HEAT grill to medium-high heat.
MELT butter in medium skillet on medium heat. Add onions; cook 8 to 10 min. or until crisp-tender, stirring occasionally.
MEANWHILE, grill steak 5 min. on each side or until medium doneness (160ºF).
STIR steak sauce into onion mixture; cook 5 min. or until heated through, stirring occasionally. Serve with steak.