01 - Combine smoked paprika, brown sugar, garlic powder, onion powder, kosher salt, black pepper, cumin, and cayenne pepper in a small bowl. Pat brisket completely dry with paper towels, then massage spice mixture evenly over entire surface of meat.
02 - Arrange sliced onions and smashed garlic cloves across bottom of slow cooker insert, creating an even bed. Place seasoned brisket on top with fat cap facing upward.
03 - Whisk together ketchup, apple cider vinegar, Worcestershire sauce, brown sugar, molasses, smoked paprika, garlic powder, onion powder, chili powder, black pepper, and salt until sugar dissolves completely.
04 - Pour half the prepared BBQ sauce over brisket, spreading evenly. Carefully pour beef broth around sides of meat, avoiding direct washing of spice rub.
05 - Cover with lid and cook on low setting for 8 hours until brisket pulls apart easily with fork. The internal temperature should reach 195-205°F for optimal tenderness.
06 - Transfer cooked brisket to cutting board using sturdy spatulas. Let rest 10 minutes before slicing against grain into thin strips or shredding with forks.
07 - Skim excess fat from remaining cooking liquid. Serve brisket with reserved BBQ sauce either brushed generously over meat or passed on the side. Top with reserved onions if desired.