London broil is actually a cooking method rather than a cut of meat. Butchers commonly label several cuts of meat as London broil, however, so the slab of beef in your refrigerator is likely a top round roast or flank steak. These cuts tend to be tough when cooked quickly, but smoking a London broil cut produces a tender, juicy dish. One key is to marinate the meat prior to placing it in your smoker.
Combine the onion, vinegar, Worcestershire sauce, soy sauce and coffee in a large glass or plastic container. Avoid using a metal container, as the acid in the marinade may react with metal. Alternatively, combine ingredients in a large zip-top baggie.
Place the meat in the marinade and cover. Refrigerate overnight or up to 2 days, turning the meat in the marinade several times to ensure that the entire cut is well-marinated.
Preheat the smoker to 225 to 250 degrees Fahrenheit and add wood to it. Place the meat on the smoker once it begins to smoke.
Cover the smoker and cook the meat for approximately 30 minutes per pound, until the internal temperature reaches 145 degrees Fahrenheit. Check the temperature of the meat by placing a meat thermometer into the thickest part of the meat.
Remove the meat from the smoker. Place it in a shallow pan and cover it with foil. Allow the meat to rest for about 20 minutes so that it reabsorbs and redistributes any juices that seep out.
Cut in thin slices at an angle against the grain and serve hot.