Preheat. Preheat your smoker to 275 degrees F using your favorite hardwood. My preferred combination for brisket is an equal mix of oak and cherry.
Trim the brisket flat. Use a sharp knife to trim off most of the fat cap from the top of the brisket and all of the silver skin from the underside of the brisket.
Inject the liquid into the meat. In a jar, combine the beef stock and the Worcestershire sauce. Use a meat injector to distribute the injection every 2-3 inches throughout the brisket flat.
Season. Sprinkle Hey Grill Hey Beef Rub equally across all sides of the brisket flat.
Smoke the brisket flat. Place the brisket flat on the smoker, close the lid, and cook for 4-5 hours, or until the internal temperature of the meat reaches 165 degrees F.
Wrap. Remove the meat to a large sheet of butcher paper. Drizzle the brisket flat with 1/4 cup of melted beef tallow and wrap tightly, tucking as you wrap to ensure there are no big air pockets or exposed pieces of brisket.
Continue smoking. Return your wrapped brisket flat to the smoker, close the lid, and continue smoking at 275 degrees F for about 2-3 hours, or until the meat is very tender. The thermometer probe should slide in and out like it's softened butter and will likely read around 200 degrees F.
Rest. Remove the smoked brisket flat from the smoker to a baking sheet and allow it to rest for at least 45 minutes or up to an hour.
Slice and serve. Once the meat is well rested, open up the paper and slice against the grain. I like to shoot for slices that are about 1/4 inch thick. Serve with BBQ sauce, pickles, peppers, and other tasty sides.