To give your London Broil the best possible flavor, marinate it overnight in a mixture of balsamic, garlic, lemon, mustard, dried herbs, oil, and just a bit of Worcestershire sauce and soy sauce. You'll notice that there’s no salt in the marinade; the soy and Worcestershire provide enough salt to season the meat well, plus they give it an extra tang of umami that will entrance the dinner table. (They'll ask what the secret ingredient is; it's up to you whether you want to tell them or not.) Once the meat is marinated and ready to grill, it only takes about 20 minutes to cook. For the crustiest char, make sure your grill grate is about four inches over glowing coals.
