London Broil, despite what you might find at the local meat market, is not a cut of beef but rather a method of cooking a steak. It was one of the first recipes to become popular in early restaurants in the United States and so the name London Broil became synonymous with a cut of meat. Where this dish originated is unknown. It certainly didn't start in England, where the term "London Broil" has no meaning. Culinary legend James Beard believed that this dish started in Philadelphia, but the actual origin is lost to history.
Originally London Broil was made with a flank steak, but over the years, the name has been applied to almost any lean, thick cut of beef. Generally, you will find London Broil being marketed as a steak or small roast that comes from the sirloin or round sections of beef. This, of course, makes the whole issue very confusing.
To make matters worse, the original method of the London Broil was a flank steak, pan fried to medium rare, cut cross grain and served. This method is perfect for a flank steak because it becomes very tough if cooked too long and by cutting it into strips you make it easy for even the dullest of teeth to get through. The problem is, this really isn't even a recipe.
Later the method was changed to include marinating the flank steak and then grilling or broiling it. This gives the name make a little more meaning. But here the origins get even more confusing. The marinade traditionally used for London Broil has ranged anywhere from a simple mixture of olive oil with salt and pepper to a wide collection of ingredients. You need to remember that chefs in earlier days tended to mix seasonings, sauces, and marinade more from what was on hand than from a specific recipe. To get a good marinade for London Broil try a mixture of soy sauce, olive oil, garlic, ginger, balsamic vinegar, and honey. This gives it the basic flavors that make beef great.
From here you need to grill the marinated flank steak hot and fast and to no more than medium doneness. Overcooking will make the meat tough no matter how long it was marinated. When the steak is cooked, remove it from the grill, cover and allow it to rest for about 5 to 10 minutes. Carve the meat across the grain, and serve in thin strips for maximum tenderness. It's great on mashed potatoes (a traditionally favorite side dish). If you've been paying attention, you will have noticed that most recipes that involve flank steak are prepared this way, from traditional fajitas too, well, most anything with flank steak. This is generally a tough cut of meat, but it has great flavor and if prepared it right, people will love it.
Okay, now for all those other things called London Broil. These cuts all have something in common; they are lean and tend to be tougher, so the same rules apply. You might find "London Broil" in anything from a 1-inch cut to a 4-inch roast. Marinate for 2 to 3 hours per inch and grill to no more than medium. On the thick roasts, you will want to grill it directly for about 2 to 4 minutes per side then indirectly for up to 30 minutes. The internal temperature should not pass 135 degrees F/55 degrees C. Allow thinner cuts to rest for about 5 minutes and whole roasts to rest for 10 minutes. Resting allows the meat to relax and the juices to flow back. Carve the London Broil across the grain and serve. It's a great way to get a really good meal out of a less expensive cut of meat.