London broil usually comes from the top round, which is at the hind side of the animal. Some butchers call flank steak or cuts from other regions London broil, too, especially when these cuts are too small to constitute a true "roast." London broil actually refers to a cooking method, not just a cut. To prepare London broil, the slab of meat is marinated and grilled or broiled, but it can also be seared in a pan and finished in a hot oven. To serve, slice the meat thinly and add a gravy or a sauce. The temperature at which you cook the meat depends on the method of preparation you choose.
All cuts that are sold as London broil benefit from high-heat cooking methods that sear the outside and leave the steak medium-rare to medium inside. Set the grill to medium heat after oiling the grates so the steak doesn't stick. Medium is equivalent to approximately 325 to 375 degrees Fahrenheit. Grill the London broil, and turn it once during cooking, for 14 to 18 minutes, depending on its size and thickness.
Broiling creates some of the same char on the outer edges of the meat that grilling does, without your having to light the grill and cook outdoors. Drain excess marinade off the meat and place it on a broiling pan. Set the broiler to high and slide the pan into the oven so it sits on the top rack that's set about 4 inches from the heat source. Cook for 4 to 5 minutes per side, or until a meat thermometer inserted reads 140 to 145 degrees F.
Place a grill or heavy saute pan -- preferably cast iron -- over a burner set to medium-high heat, and add enough oil to coat the bottom. Preheat your oven to 375 degrees F. Add the meat and cook for 2 to 3 minutes per side. Slide the entire pan into the oven; if you have a pan with a plastic handle, transfer the steak to a baking sheet first. Roast the meat for about 10 minutes; check it often to be sure it doesn't overcook.
Let the London broil come to room temperature before you grill, broil or sear it so that it cooks more evenly. Once the London broil is done cooking, let it rest for about 10 minutes so the juices settle and don't spew out when you slice into it. Cut the steak into thin strips, against the grain, to make it less chewy.
Quick marinade ideas include soy sauce, minced garlic, chopped scallions and sesame oil; fresh thyme, balsamic vinegar and olive oil; or dried Italian herbs, Dijon mustard, lemon juice and olive oil. Allow the meat to marinate for about 8 hours prior to cooking.