Trim excess fat from the brisket, leaving a thin layer for flavor and moisture. Season the brisket generously with salt and pepper.
Drizzle one side of the brisket with half of the yellow mustard. Using your hands, rub the mustard into the brisket evenly.
Flip the brisket over and do the same on the other side with remaining mustard.
In a bowl, mix together the beef broth, barbecue sauce, Worcestershire sauce, brown sugar, apple cider vinegar, Dijon mustard, smoked paprika, garlic powder, onion powder, and cayenne pepper (if using).
Place the sliced onions and minced garlic in the bottom of the crockpot. Lay the seasoned brisket on top of the onions and garlic.
Pour the sauce mixture over the brisket, ensuring it's evenly coated.
Cover the crockpot with the lid and cook on low heat for 7-8 hours, or until the brisket is tender and easily shreds with a fork.
Once the brisket is cooked, remove it from the crockpot and let it rest for a few minutes before slicing against the grain.
Serve the sliced brisket with the onions and sauce from the crockpot.