The term "London broil" can confuse, because technically it is a method of grilling and slicing the meat rather than a specific cut of beef. However, supermarket cuts labeled "London broil" are most often cut from the flank. Both flank and top round offer good flavor, but they differ in texture.
A London broil cut from the flank comes from the animal's underbelly, just in front of the rear legs. Top round, meanwhile, is cut from the muscles surrounding the rear leg bones.
Size and shape
London broil/flank is typically long and flat, about two inches thick. Top round is a rounder, bulkier muscle, typically about six inches in diameter.
London broil/flank displays visible muscle striation and contains a good deal of tough connective tissue and ample fat. Top round contains much less connective tissue and fat, which gives it a uniform, smooth surface.
London broil/flank can be tough and chewy, owing to its long muscle fibers and connective tissue. Top round is more tender, similar to sirloin, though not as tender as a prime rib or tenderloin.
London broil/flank steak is usually grilled or broiled, but can also be roasted or braised. Because of its size, top round lends itself well to roasting.
Serve London broil in very thin slices, making sure to cut across the grain to reduce chewiness. Top round should also be cut across the grain, sliced as thick as you wish.