London broil is a preparation, not a cut of meat. The customary procedure is to marinate, bake or broil to medium-rare and serve in thin slices. Traditional London broil is made with marinated flank steak. Top round is often substituted for the tasty but tough flank. Marinades vary from plain salt and pepper to subtle blends of wine and spices or barbecue-like vinegar and brown sugar. Broiling beyond medium rare risks a tough entrée, so well-done London broil requires a more tender cut of meat than either flank or top round.
Cut diagonal lines about 1/4 inch into the roast with a sharp knife.
Place the roast and marinade in the container, and marinate for 2 to 4 hours in the refrigerator. Sirloin is a tender cut that does not require marinating overnight.
Remove the roast from its container, and set it on the broiler pan. Allow the roast to sit until it comes to room temperature. Pour the remaining marinade over the roast into the pan or put 1/2-inch of water in the bottom to provide moisture.
Turn the broiler on high, and position the rack so the top of the meat sits 3 inches from the heating element.
Broil the roast for 5 to 7 minutes on the first side until it sears and begins to crisp. Remove the roast, turn it with forks, baste it with pan juices, then return it to the broiler. Broil the second side until it begins to crisp.
Lower the broiler rack to the bottom setting or turn the broiler to low if it has more than one setting. Cover the roast with aluminum foil and return it to the broiler until its internal temperature reaches 155 to 160 degrees Fahrenheit, from 5 to 10 minutes.
Remove the roast from the broiler and let it sit for 10 minutes under the foil before serving. e.