Lightly score edges of steak at 1-inch intervals. Preheat grill or broiler. Combine butter, parsley, onion, Worcestershire sauce, pepper and mustard in a small saucepan. Heat, stirring continually, over low heat, until butter melts. Reserve ΒΌ of the mixture. Place steak on grill or broiler pan. Brush steak with butter mixture. Cook, basting frequently with butter mixture, about 6 minutes per side for medium. Place steak on a serving platter. Cut thin slices across the grain. Drizzle reserved butter mixture over steak.