Grill a London broil precisely to medium-well by using a meat thermometer. This is a flavorful and lean cut of meat that sits on the underside of the cow below the shortloin and sirloin cuts. London broil is a value-priced piece of beef. It becomes tender when you marinate it correctly and carve it into thin slices.

Prepare your London broil for the grill. Defrost frozen beef in the refrigerator, not at room temperature. Leave London broil in the refrigerator until you are ready to place it on the grill. Trim off the excess fat with a sharp knife to reduce the saturated fat content of your meal and to help prevent flare-ups while it grills.

Marinate your London broil to tenderize it and add flavor. Let it soak in the liquid for six to 24 hours. Turn the London broil several times while marinating to let it permeate the meat evenly. For each pound of London broil use at least ¼ cup of a liquid marinade. Ensure your marinade tenderizes the meat by adding an acidic ingredient to your marinade recipe such as Italian dressing, lemon juice or wine. Other marinade ingredients can include soy sauce, salsa, fresh garlic and brown sugar. Save a bit of unused marinade to create a dipping sauce or gravy.

Prepare your grill by taking it up to medium heat. Preheat it if you are grilling with gas. If you are using a charcoal grill, burn down the charcoal for about 30 minutes, until the coals are glowing and a thin covering ash sits on top when you use a conventional grill.

Take the London broil out of the refrigerator and pat it dry with paper towels to facilitate browning. Insert a heat resistant meat thermometer into the side of the London broil at its thickest point, until the tip reaches the center of the meat.

Place the London broil on top of the grill grate to cook uncovered. Turn the London broil only once with tongs about halfway through the cooking time.

Take your London broil off the grill when the temperature on the thermometer reads 5 degrees less than 145 degrees Fahrenheit -- the final temperature. Let it stand under a tent of aluminum foil until the temperature rises to the final temperature. Expect the meat to be brown and fully cooked on the outside and pink in the center.