In a large bowl, coat the braising steak all over in the BBQ rub. Heat the oil in a large, cast iron pan and sear the beef, on high heat, for 2 minutes on each side, until brown all over, then remove from the pan. We're not trying to cook the beef through at this point.

Add the beef stock, apple cider vinegar, ketchup, Worcestershire sauce, garlic, black pepper and onion granules to a slow cooker and stir to combine. Add the beef to the slow cooker and sprinkle with any remaining BBQ rub. Cover and cook on HIGH for four hours.

Once the beef is cooked, transfer to a large bowl and pull apart using two forks. Mix in little of the remaining sauce from the slow cooker to the beef and reserve the rest to use for dipping.

Make your tacos. Heat a little oil in a non-stick pan, on low heat, add the mini tortillas (we were able to fit 3 in the pan at a time), mozzarella, BBQ beef, onion, coriander and jalapeños. Fry for 2 minutes on each side, then remove from pan. Repeat for remaining tacos.