"London broil" refers to a classic dish of marinated flank steak, but that doesn't stop supermarkets from labeling any leftover tough cut of meat "London broil." The classic London broil technique comprises marinating a flank steak in dry red wine, oil and aromatics, then grilling it and slicing it across the grain. Although you can't grill a London broil indoors, you can oven-broil it -- the same as grilling, except the heat radiates from above instead of below. The oven makes cooking London broil to medium-rare a foolproof task, and takes a fraction of the time grilling does.
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Marinate the flank steak for about 4 hours in an oil-based marinade. Equal parts acid and oil, plus herbs and spices to taste constitute a basic oil-based marinade.
Classic London broil marinade consists of 2 parts dry red wine to 1 part each oil, red wine vinegar, Worcestershire sauce and sugar, along with a few cloves of minced garlic and kosher salt to taste. You can add most other herbs, such as thyme and rosemary, and additional aromatics, such as shallots and ginger, as long as you use equal parts oil and acid as the base.
Take the flank steak out of the marinade after about 4 hours in the fridge. Wipe off the excess marinade and let the flank steak sit at room temperature in a shallow dish for 45 minutes to 1 hour.
Set the oven rack 4 to 6 inches below the oven broiler and place a cast iron skillet on it. Look the skillet over for any food debris and wipe it out, if necessary.
Turn the broiler on and let the skillet heat for about 30 minutes. Coat the flank steak on both sides with high-temperature oil, such as canola, sunflower or peanut oil, and season it to taste with freshly ground black pepper, if desired.
Pull the oven rack out -- don't take the skillet out of the oven. Lay the flank steak in the skillet using tongs and slide the rack back in.
Broil the flank steak for 5 minutes and pull the oven rack out. Lift the flank steak from the skillet using the tongs and place it on a plate. Slide the rack in and turn the oven off -- don't take the skillet out of the oven until it cools to room temperature, about 1 hour.
Rest the flank steak for about 10 minutes and transfer it to a cutting board. Slice the flank steak on the bias -- or at a 45-degree angle -- across the grain into 1-inch-wide strips and serve.