1. Score the flank steak with a sharp knife, about 1/8-inch deep, in a diamond pattern; set aside.
2. Mash the garlic with the mustard.
3. Stir in the whiskey and oil.
4. Place steak in a zipper type plastic freezer bag. Pour mixture over the steak and refrigerate overnight.
5. Set out at room temperature for 1 hour before cooking. Remove steak from marinade, reserving marinade.
6. Grill, using charcoal or gas grill. Cook over high heat 3 to 5 minutes per side. (Or cook under broiler in oven.)
7. Heat reserved marinade in small pan to a boil.
8. Slice the steak by cutting across the grain into 1/4-inch thick slices. Serve with heated marinade as a sauce.