When you need only one ingredient and one pan for a dish -- such as London broil and a skillet -- make sure both are high quality. When it comes to the skillet, go for cast iron or tri-ply stainless steel. Both are thick and heavy, which means they don't have hot spots and cook more evenly than thin-gauge metals like aluminum. Choose a flank steak -- the traditional cut used for London broil -- with a bright red hue, which indicates freshness and the best odds for tenderness at the table.
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Marinate flank steak overnight. It's that simple; there aren't any rules when it comes to marinating London broil, just combine the spices and herbs that suit your tastes in a marinade that uses oil as the base. You could marinate flank steak in a couple tablespoons of Worcestershire sauce and half a cup of olive oil, for example, and have a tender, flavorful London broil the next day.
Take the flank steak out of the refrigerator about 30 to 45 minutes before you want to cook it and let it sit at room temperature. Leave the steak in the marinade while it warms up.
Heat a few tablespoons of olive oil in a cast-iron skillet or tri-ply stainless-steel skillet on the stove for 3 or 4 minutes. Use medium heat.
Pat the flank steak with a paper towel and lay it in the pan. Cook the flank steak until you see the edges along the bottom of the meat start to tighten, or when they appear to shrink a little. It should take around 3 full minutes, but it varies depending on the size of the steak.
Turn the flank steak over with tongs and let it cook another 2 to 3 minutes. The flank steak should be about rare at this point.
Cook the flank steak another 3 or 4 minutes for medium rare, bringing the total cooking time to 9 or 10 minutes. Turn the steak occasionally with tongs so it cooks evenly on both sides. If you want a medium flank steak, cook it for a total of 12 minutes. You can check the steak's internal temperature for confirmation by inserting an instant-read meat thermometer in the center. Medium rare is between 125 and 130 degrees Fahrenheit, and medium is around 135 F. Take the steak out of the pan when checking the temperature.
Transfer the flank steak to a carving board and let it rest for about 5 minutes. Cover the steak with a piece of foil, but not tightly, just enough to keep some steam around it.
Find the direction of the grain, or the meat fibers, and cut the flank steak crosswise across it. Cutting across the grain is more important than marinating for making the steak easy to chew. You also want thin, broad slices, so hold the knife at an angle when you cut and space each slice about 1/4 inch apart. Serve immediately.