In a blender, combine all ingredients except London broil. Process until smooth. Place London broil in a non-reactive container and pour marinade over top, turning it to coat. Refrigerate for at least 4 to 6 hours before cooking.
Preheat grill to high. Place London broil on white hot grill, and cook for 4 to 5 minutes on each side, flipping steak 4 times (cooking time will vary with thickness of steak). Let steak rest for at least 10 minutes before thinly slicing against the grain of the meat and on a bias for wide but thin slices. Garnish with fresh cilantro.