PREHEAT barbecue to medium-high heat. Mix butter, cheese and onion until well blended; cover. Refrigerate until ready to use.
GRILL steaks 10 to 12 min. for medium-rare doneness (140°F) or slightly longer for medium doneness (160°F), turning occasionally and brushing with barbecue sauce.
REMOVE and discard bones from steaks; carve meat into slices. Serve topped with butter mixture.