London broil and flank steak are actually the same cut of beef. The difference between them is in how they’re prepared and cut for serving. Because flank steak comes from the steer’s lower hindquarters, it’s tough. To tenderize, both London broil and flank steak are usually marinated overnight. While flank steak is cooked whole, London broil is cut into large pieces and then cooked and sliced across the grain.
Although shoppers may find top round steak or roast labeled “London broil,” traditionally it is flank steak, known for its flavor and tooth. Because this tough cut of meat requires long cooking times to be tender, many cooks choose to marinate it, making the most of flank and other tough cuts like brisket. Also, because flank steak is very thin, it tends to dry out during cooking. Marinade infuses flavor at the same time it raises fat content to keep the meat juicy. Marinating also reduces cooking time because the flank steak is already tenderizing. For both applications, the meat is seared before finishing. While flank steak is commonly grilled to make fajitas and carne asada, London broil is usually cooked under an oven’s broiler, about five minutes a side, then rested another five minutes before slicing.