1. In a medium bowl, whisk together the Worcestershire sauce, soy sauce, brown sugar, mustard, vinegar, garlic, salt and black pepper until smooth. Whisk in the oil.
2. Place the flank steak inside a gallon-sized zipper-lock bag with the remaining marinade. Press out air, seal the bag, and allow the meat to marinate, turning occasionally, for at least 4 hours and up to 24 hours.
3. Prepare the grill for direct grilling over a medium-high fire (600°F) with an indirect cooking zone as well. A charcoal fire is preferred. Remove the flank steak from the marinade and blot dry with paper towels. Brush both sides with a bit more avocado oil and season with a bit more salt and pepper.
4. Grill the steak over direct heat until charred on both sides, 60 to 90 seconds per side. Move the seared steak away from the direct fire to the indirect side, close the lid, and gently roast it until they reach your desired internal temperature (such as 125°F for medium-rare), turning every 90 seconds.
5. Transfer the meat to a cutting board, loosely tent with foil and let rest for 10 minutes. Slice thinly against the grain, making sure to pour any of the accumulated juices over the meat before you serve it!
