01 - Combine brown sugar, paprika, kosher salt, black pepper, garlic powder, onion powder, cayenne pepper, and dried thyme in a small bowl. Mix thoroughly until evenly incorporated.
02 - Pat the beef brisket completely dry with paper towels. Massage the spice rub mixture over the entire surface of the meat, ensuring even coverage on all sides.
03 - Arrange sliced onions and smashed garlic cloves evenly across the bottom of the slow cooker insert. Position the seasoned brisket on top with the fat cap facing upward.
04 - Pour beef broth around the sides of the brisket, avoiding direct pouring over the top to preserve the spice rub coating.
05 - Cover with lid and cook on low setting for 8 hours. The brisket is done when it shreds easily with a fork and offers no resistance.
06 - Using tongs, carefully transfer the cooked brisket to a cutting board. Allow it to rest for 10-15 minutes before slicing. Skim excess fat from the cooking liquid if preferred.
07 - Combine BBQ sauce, apple cider vinegar, Worcestershire sauce, and 1/2 cup of reserved cooking juices in a small saucepan. Simmer over medium heat for 5-10 minutes, stirring occasionally until slightly thickened.
08 - Slice the brisket against the grain into thin strips or shred using two forks. Drizzle warm BBQ glaze over the meat and serve immediately.