In a small bowl, whisk together all ingredients (except the beef).
Lightly score the beef on both sides and place in a shallow dish or large ziploc bag. Add the marinade to the dish/bag and let sit in the fridge (covered if using a dish) for 1 to 24 hours.
Remove marinated meat from fridge and let sit at room temperature for 1 hour. After 30 minutes, turn the broiler on high and place your cast iron skillet under the broiler. Let it preheat for 30 minutes.
Quickly remove the skillet from the oven and carefully place the london broil in it and return to the oven for 5 - 7 minutes per side. Once cooked, let the london broil rest for 7 minutes before slicing to keep the juices in. I like to make an aluminum foil tent to keep it from getting cold. When ready, slice against the grain and enjoy!