Place the flank steak in a large, plastic resealable bag.
In a large, glass measuring cup, combine red wine vinegar, red wine, olive oil, Worcestershire, soy sauce, mustard, essence, salt, pepper, and garlic. Pour the marinade into the bag with the steak, seal and place inside a large, glass baking dish to prevent possible spills. Marinate in the refrigerator for 24 hours (or at least 6 hours).
Preheat a gas or charcoal grill to high heat. (Alternately, preheat the broiler). Remove the steak from the marinade and pat dry with paper towels. Place the steak on the grill (or under the broiler) and cook to desired doneness, about 6 minutes per side for rare. Transfer the steak to a cutting board and let stand for 5 minutes before carving.
Slice across the grain into thin diagonal slices. Serve steak with sauce.
Yield: 4 servings