Pour marinade into large resealable plastic bag. Add steaks; turn to coat. Seal bag. Refrigerate 30 min. to marinate.
Meanwhile, mix butter, cheese and onion until well blended; cover. Refrigerate until ready to use.
Preheat grill to medium heat. Remove steaks from marinade; discard marinade. Grill steaks, uncovered, 10 to 12 min. for medium-rare to medium doneness, turning occasionally. Remove steaks from grill. Remove bones; cut steak into thin slices. Top with the butter mixture.