In a small bowl, mix smoked paprika, chili powder, brown sugar, garlic powder, onion powder, cumin, salt, pepper, and cayenne (if using).
Pat brisket dry and rub the seasoning mixture all over, pressing it into the meat.
Optional: Heat a skillet over medium-high heat with a tablespoon of olive oil and sear brisket 2 to 3 minutes per side until browned.
Add sliced onion and minced garlic to the bottom of a 6-quart slow cooker.
Whisk beef broth, Worcestershire sauce, apple cider vinegar, Dijon (if using), and 2 tablespoons barbecue sauce. Pour the mixture around the brisket.
Cook on low for 8 to 10 hours, until the brisket is very tender and pulls apart easily.
Remove brisket and rest 15 to 20 minutes, tented with foil.
Skim excess fat from slow cooker liquid if needed. Stir in remaining barbecue sauce.
Slice brisket against the grain, or shred with two forks. Toss with sauce.
Optional: Broil brisket on a foil-lined baking sheet with sauce for 3 to 6 minutes for caramelized edges. Serve with extra sauce.