Preheat the oven to 300°F.
Combine the Worcestershire, vinegar, bouillon and 1 tablespoon of brown sugar in a microwave-safe cup or bowl. Microwave for 30 second or until the cube has dissolved. Then, let it chill to room temperature.
Clean the meat under cool water and completely pat it dry with a paper towel. Using an injector, inject it with a ½ cup of the microwaved mixture. Be sure to inject all sides and points of the meat.
Drizzle enough oil on the meat to cover it, sprinkle it liberally with the barbecue rub and then let it sit until it comes to room temperature.
While the meat rests, prepare a large skillet with enough oil to cover the bottom of the pan over medium-high heat. Add the meat to the pan and sear it on all sides, until it’s evenly browned. Once it’s browned, remove the meat and set it to the side.
Then, in a roasting pan, add the sliced onions and drizzle the mustard evenly over them. Place the meat on top of the onions, add the beer, the remaining injection liquid and 1/2 of the barbecue sauce. Then, add water until the meat is halfway covered.
Cover the pan, transfer it to the preheated oven and cook for 5–6 hours, or until the meat is fork tender. You can test for doneness by picking at the meat with a fork. If it easily pulls, it’s done.
Remove the meat from the pan and shred it into large chunks using a fork. Then, using a heavy knife or cleaver, chop it up on a cutting board until it’s fully minced.
Transfer the meat to a bowl, sprinkle with a little bit of barbecue rub and the remaining barbecue sauce and toss to combine.
To make the blue cheese sauce, combine all the ingredients in a bowl until they’re well combined.
To serve, lather both the top and bottom buns with the sauce, place the chopped beef on the bottom bun and top with the other bun.
Tip: Pan sauce from the roasting pan can be added to the meat to keep it moist inside the bowl. Also, it helps to toast the buns before assembling the sandwiches, to make a sturdier base.