01 - Combine brown sugar, smoked paprika, kosher salt, black pepper, garlic powder, onion powder, cayenne pepper, and dried thyme in a small bowl. Mix thoroughly to ensure even distribution of all spices.
02 - Pat the beef brisket completely dry with paper towels. Apply the spice rub mixture evenly over the entire surface of the meat, pressing gently to adhere the spices to all sides.
03 - Arrange the sliced onions and smashed garlic cloves in an even layer across the bottom of the slow cooker insert. Place the seasoned brisket on top of the aromatics with the fat side facing upward.
04 - Pour the beef broth carefully around the sides of the brisket, avoiding direct pouring over the seasoned top to prevent washing off the spice rub.
05 - Cover the slow cooker with its lid. Cook on low heat setting for 8 hours until the brisket reaches fork-tender consistency and can be easily pulled apart.
06 - Transfer the cooked brisket carefully to a large cutting board. Remove any excess fat floating on top of the cooking liquid if desired. Allow the meat to rest for 10-15 minutes before slicing.
07 - Combine BBQ sauce, apple cider vinegar, Worcestershire sauce, and 1/2 cup of the reserved cooking liquid 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 with forks. Arrange on a serving platter and drizzle the warm BBQ glaze generously over the top. Serve immediately with additional sauce on the side.