Score the flank steak with a sharp knife, about 1/8-inch deep, in a diamond pattern; set aside. Mash the garlic with the mustard. Stir in the whiskey; pour mixture over the steak and refrigerate, covered, overnight. (Easiest done in a Zip-Loc bag). Set out at room temperature for 2 hours before cooking. Grill, using charcoal or gas grill. Cook over high heat 3 to 5 minutes per side, dotting each side with butter while cooking. Slice the steak immediatly by cutting across the grain into 1/4-inch thick slices. Sprinkle with salt and pepper. Makes 4 servings. Shared by Ellie Collin, Prodigy Id# CMKD93F.