London broil is a name butchers use to refer to any large and lean cut of meat like a flank steak, top round roast, or a top sirloin roast. London broil is most tender and has the richest flavor when cooked slowly in an oven, crock-pot, or slow-cooker and is especially delicious sliced over mashed potatoes or turned into fajitas. You can use a London broil for almost any recipe calling for flank steak.