To cook London broil on the grill, first bring the steak up to room temperature before placing it on an oiled rack 5 to 6 inches above the coals. For medium-rare steak, let the steak rest on each side for no more than seven to nine minutes.
Before grilling the steak, it needs to be marinated for at least four hours and up to 24. After the steak is cooked, it should be sliced into thin slices across the grain and served.
The term "London broil" prefers specifically to a type of preparation in which meat is marinated, grilled or broiled, then thinly sliced, according to Real Simple. Traditionally, flank steak is used, though London broil is sometimes incorrectly used to refer to top round steak. Slicing thinly across the grain shortens the fiber and makes flank steak more tender, but the same does not hold true for round steak, which must be cut into smaller chunks first.
The marinade for London broil usually contains something acidic such as vinegar, wine or soy sauce. Seasonings and spices are added afterwards. Traditionally, the marinades are made from anything on hand. According to G. Stephen Jones of The Reluctant Gourmet, marinades do not make steaks more tender but are highly valuable for imparting flavor.