01 - Combine brown sugar, smoked paprika, kosher salt, black pepper, garlic powder, onion powder, cayenne, and dried thyme in a small bowl. Mix thoroughly to incorporate all spices evenly.
02 - Pat the beef brisket completely dry with paper towels. Massage the spice rub mixture over all surfaces, ensuring even coverage on all sides of the meat.
03 - Arrange the sliced onions and smashed garlic cloves across the bottom of the slow cooker insert. Position the seasoned brisket on top of the aromatic bed with the fat layer facing upward.
04 - Carefully pour the beef broth around the sides of the brisket, avoiding direct pouring over the seasoned meat. Cover securely and cook on low heat setting for 8 hours until fork-tender.
05 - Transfer the cooked brisket to a cutting board and tent with foil. In a saucepan, combine BBQ sauce, apple cider vinegar, Worcestershire sauce, and 0.5 cup of the slow cooker juices. Simmer for 5-10 minutes until slightly thickened.
06 - Slice the brisket against the grain into portions or shred using two forks. Serve immediately with the warm BBQ glaze drizzled over the top, accompanied by coleslaw, pickles, and rolls.