A note from Chef Naomi Ross: A London Broil is a thick-cut steak that is generally marinated and grilled and then sliced thinly crosswise. Because of the thickness, it can often be hard to gauge doneness—leaving you with an overdone exterior and an undercooked interior. For the best results, I cook it slower on indirect heat (not over the fire) and then give it a reverse sear on direct heat (over the fire) at the end of cooking time. Definitely use your meat thermometer to test for desired doneness.