In a small bowl, whisk together the brown sugar, smoked paprika, garlic powder, onion powder, cumin, salt, black pepper, and cayenne until evenly combined.

Pat the brisket completely dry with paper towels. Rub the spice mixture generously over all sides of the meat, massaging it into the surface to form a crust.

Place the sliced onions and minced garlic in the bottom of a 6-quart slow cooker.

In a medium bowl, whisk together the BBQ sauce, beef broth, apple cider vinegar, and Worcestershire sauce. Pour half of this mixture over the onions in the slow cooker.

Place the seasoned brisket on top of the onions, fat side up if there is a fat cap. Pour the remaining sauce mixture over the top of the meat.

Cover and cook on Low for 8 to 10 hours (recommended for maximum tenderness) or on High for 5 to 6 hours, until the brisket is fork-tender and pulls apart easily.

Carefully transfer the cooked brisket to a cutting board and let it rest for 15 minutes. Slice against the grain into 1/4-inch thick pieces for traditional servings, or shred with two forks for pulled beef.

Skim the fat from the cooking liquid remaining in the slow cooker. For a thicker sauce, pour the liquid into a saucepan and simmer on the stovetop for 5 to 10 minutes to reduce. Pour the sauce over the sliced or shredded brisket before serving.