1. Trim any fat from the meat and place it into a large plastic storage bag.
2. Mix the vinegar, blacken, lemon zest and juice in a stainless bowl, then pour it into the bag. Remove as much air as you can from the bag, seal and place in the refrigerator. Let the meat marinate at least 24 hours, or, up to 3 days.
3. When you're ready to cook, make the first chopping the peppers and onion.
4. Place them into a Teflon fry pan with a little vegetable oil and cook over a medium heat until they're soft.
5. Drain the marinade into the pan, and reduce it until it's quite thick.
6. Add the cream, and stir it in while continuing to cook. This sauce will become fairly mild yet full of flavor if you use the amount of cream I've specified in the ingredients. By using less cream, or none at all you'll end up with a REAL spicy sauce.
Let this cool while you're cooking the London Broil.
7. To cook the meat, start the grill and let it get to a medium heat. Coat the meat with vegetable oil, then place on the grill. Cook about 7 minutes, then flip. Continue flipping the meat every 5 minutes of so until the internal temperature reads 110 degrees (for rare) on an instant read meat thermometer. Place the cooked meat on a rimmed platter, and cover it with aluminum foil. Let this rest at least 10 minutes.
Total cooking time will be about 20 minutes, but can vary greatly by the heat of the grill, and the thickness of the meat. When cooking a thick piece of meat, I'll sometimes cover the meat during about half the total cooking time. This help cook the interior of the meat without burning the outside.
8. While the meat is resting, you've got plenty of time to reheat the sauce, and cook or reheat any of the sides you'll be serving with this.
9. To Serve:
Slice the meat real thin, across the grain, then drizzle some of the sauce on top.