Technically, London broil is not a cut per se, but a method of cooking that involves marinating, then grilling or broiling a tough cut of meat (traditionally flank steak) to medium rare, and cutting it against the grain into thin slices. That said, in the supermarket you will often see steaks labeled London broil. This name most commonly refers to two different boneless cuts, the top round (pictured), which comes from the upper leg, and the chuck shoulder steak. We generally use top round because it is widely available and simpler to carve than chuck shoulder steak, which has better flavor but some gristle.
London Broil with Soy Citrus Mayonnaise
Grilled Balsamic-Marinated London Broil with Red Onions