PREHEAT barbecue to medium-high heat. Grill steaks 14 min. or until medium doneness (160ºF), turning after 7 min. and brushing with 1/4 cup of the barbecue sauce.
MEANWHILE, cook and stir onions and mushrooms in butter in large skillet on medium-high heat 2 to 3 min. or until vegetables are tender. Stir in remaining 1/4 cup barbecue sauce and the brandy. Reduce heat to low; cook an additional 5 min. or until heated through, stirring occasionally.
SERVE steaks topped with the barbecue sauce mixture.