01 - Rub the beef brisket thoroughly with kosher salt and freshly ground black pepper, coating all sides evenly.
02 - Arrange the sliced onions and minced garlic in an even layer at the bottom of the slow cooker insert.
03 - Combine ketchup, apple cider vinegar, brown sugar, Worcestershire sauce, Dijon mustard, smoked paprika, garlic powder, onion powder, and cayenne pepper in a medium bowl. Whisk until the brown sugar dissolves completely and the mixture is smooth.
04 - Place the seasoned brisket on top of the onion bed in the slow cooker. Pour the prepared BBQ sauce over the meat, ensuring complete coverage.
05 - Cover with the lid and cook on low setting for 8 hours until the brisket is fork-tender and easily shreds.
06 - Carefully transfer the cooked brisket to a cutting board. Let it rest for 10 minutes to allow the juices to redistribute.
07 - Skim any excess fat from the cooking liquid. For a thicker consistency, transfer the sauce to a saucepan and simmer for 10 minutes to reduce.
08 - Slice the brisket against the grain into thin strips. Serve immediately with the reduced BBQ sauce spooned generously over the top.