London broil is not a cut of beef, but instead a way to prepare it. London broil is usually made with a top round steak, but can also be done with flank steak or sirloin steak. The secrets to a great london broil are a flavorful marinade and slicing thinly against the grain. Leftover london broil makes great cold roast beef sandwiches. It's also great served with some homemade brown gravy.