The rich and bold flavor of marinated flank steak calls for hearty sides like Baked Potatoes with Onion Sour Cream and Green Bean and Yellow Pepper Salad.
In a 9-by-13-inch baking dish, whisk together garlic, 1/2 teaspoon each salt and pepper, apple juice, whiskey (if using), mustard, oil, Worcestershire, and hot sauce. Add steaks, and turn to coat; marinate at room temperature, turning occasionally, 1 hour, or cover with plastic wrap and refrigerate up to 1 day.
Lift steaks from marinade onto a platter; set aside. Make sauce: Place reserved marinade in a large saucepan. Bring to a boil over high heat; cook until thickened and reduced to about 1 cup, 20 to 25 minutes. Season with salt and pepper.
Meanwhile, heat grill to high; lightly oil grates. Place steaks on grill. Cook, turning once, until well browned and cooked to desired doneness, 6 to 8 minutes per side for medium-rare (130 to 135 degrees on an instant-read thermometer). Transfer steaks to a cutting board, and tent with aluminum foil. Let rest 10 minutes before thinly slicing against the grain. Serve with sauce.
To read the internal temperature of steak, insert an instant-read thermometer horizontally until tip reaches the thickest part.