PLACE steak in shallow dish. Pour 1/2 cup marinade over steak; turn to evenly coat both sides of steak. Refrigerate 30 min. to marinate, turning occasionally.
HEAT grill to medium heat. Remove steak from marinade; discard marinade. Grill steak 13 to 16 min. for medium-rare to medium doneness, turning occasionally. Remove steak from grill; cover to keep warm.
COOK and stir mushrooms, onions and garlic in large skillet on medium-high heat 5 min. Stir in remaining marinade; simmer on low heat 5 min.
CUT steak across the grain into thin slices. Serve topped with mushroom mixture.