Combine rub ingredients in a small bowl. Generously coat brisket with lots of black pepper on all sides then the spice rub, massaging the spices into the meat. Cover with plastic wrap and let sit overnight in the fridge.
Preheat a smoker to around 250°F and set up for cooking over indirect heat by placing coals and wood chips to one side. Place 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.