Combine the rub ingredients in a small bowl. Generously coat the brisket with lots of black pepper on all sides, followed by the spice rub, massaging the spices into the meat. Cover with plastic wrap and let sit overnight in the fridge.
Pre-heat a smoker to around 250°F and set up for cooking over indirect heat by placing coals and wood chips to one side. Place the brisket on the other side and cook, fat side up, for 4 hours.
Pull the meat off and slather with the grated onion. Using a double layer of foil, create a pouch/packet for the brisket. Add apple juice and beer and tightly seal the foil.
Place back in the smoker for another 2 hours, adding a handful of chips and 12 or so coals. After 2 hours, open the lid and let the brisket sit in the packet for another hour.
Combine all ingredients for RR Smoky Beef BBQ Sauce in a small saucepot and cook over a low bubble for 15-20 minutes. Reserve.
Stir the ingredients for the Horseradish Sauce together in a small bowl to combine. Reserve.
To serve, thinly slice the brisket on a large cutting board. Add some RR Smoky Beef BBQ Sauce and give everything a rough chop to combine.
Assemble sammies by placing a pile of brisket on a roll bottom. Add a sprinkle of cheese, then some Red Cabbage Slaw. Slather the roll tops with horseradish sauce and replace tops.