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 the meat.
02 - Arrange sliced onions and smashed garlic cloves across the bottom of a 6–7 quart slow cooker. Position the seasoned brisket on top of the onion bed, 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 in a medium bowl until sugar dissolves completely.
04 - Pour half the prepared BBQ sauce evenly over the brisket. Add beef broth around the sides of the meat, avoiding direct pouring over the seasoned surface. Reserve remaining sauce for serving.
05 - Cover slow cooker with lid. Cook on low setting for approximately 8 hours until brisket probes tender and shreds easily with a fork.
06 - Transfer cooked brisket carefully to a cutting board using large spatulas. Let meat rest for 10 minutes before slicing against the grain or shredding with forks.
07 - Skim excess fat from cooking liquid. Serve brisket with reserved BBQ sauce either brushed generously over sliced meat or offered on the side for individual portioning. Garnish with additional cooked onions if desired.