Prepare Sauce before grilling steak.
In saucepan, cook onion in butter until tender.
Add remaining ingredients.
Heat, stirring occasionally.
Keep warm to serve over sliced meat.
Place steak on grill 3 to 4 inches from hot coals.
Grill 5 minutes on each side for medium rare.
Thinly slice meat diagonally across the grain.
Serve with Sauce