Prepare the rub by combining all the ingredients and then rub all over brisket on both sides.
Cut four onions in rings and layer on bottom of a slow cooker pot on high.
Place brisket over onions.
Combine all the sauce ingredients, and pour over brisket.
Cover slow cooker with lid and let cook 6-7 hours on high.
After 6-7 hours of cooking, remove brisket. Use two forks to shred the meat. Return to pot, and mix meat into the sauce that remained in the pot. Cook for a few more minutes.