London broil is not a cut of beef, but instead a way to prepare it. London broil is usually made with a top round steak, but can also be done with flank steak or sirloin steak.
The secrets to a great London broil are a flavorful marinade and slicing thinly against the grain. Leftover London broil makes great cold roast beef sandwiches. It's also great served with some homemade brown gravy.
In a small bowl whisk together the vinegar, oil, garlic and rosemary. Place the beef on a plate and poke both sides all over with a fork. Transfer to a plastic freezer bag and pour in the marinade. Seal and refrigerate overnight.
Turn broil on high. Remove the London broil from the marinade and pat dry. Salt generously, and rub in the freshly ground black pepper on both sides. Place on a broiler pan or other oven-safe pan.
Broil about 8 inches under the flame for approximately 6-7 minutes per side for medium rare - an internal temp of 130 degrees F.
Transfer to a plate and cover loosely with foil; let rest for 10 minutes. To serve, cut against the grain into very thin slices. Serve hot, topped with any juices from the plate on which it rested.