The ground beef for this recipe should be equal parts top sirloin and top round, and 80 percent lean. The chef of The "21" Club in New York, New York, Michael Lomonaco advises, "Handle the beef as little as possible - in other words, don't knead the meat - because overhandling changes its texture, moistness, and flavor." The less compacted the burger, the juicier it will be. For the authentic "21" Club burger, you should use the peasant-style bread that Michael calls for in his recipe, but failing that, use a good quality crusty bread and avoid the usual burger buns.