01 - Arrange the sliced onions and minced garlic evenly across the bottom of the slow cooker insert.
02 - In a small bowl, whisk together the beef broth, barbecue sauce, tomato paste, brown sugar, Worcestershire sauce, smoked paprika, cumin, salt, black pepper, and chili powder until fully blended.
03 - Place the beef chuck roast on top of the onion bed. Pour the sauce mixture over the roast, ensuring the meat is well coated. Cover and cook on low heat for 8 hours, or until the meat is tender and easily pulls apart.
04 - Remove the cooked beef from the slow cooker and transfer to a cutting board. Use two forks to shred the meat, discarding any excess fat. Skim the fat from the remaining liquid in the cooker, return the shredded beef to the sauce, and stir to combine.
05 - In a large bowl, whisk together the mayonnaise, apple cider vinegar, sugar, salt, black pepper, and celery seed until smooth.
06 - Add the shredded cabbage and carrots to the dressing. Toss thoroughly to coat all vegetables. Refrigerate until ready to serve.
07 - Lightly toast the sandwich buns if desired. Pile a generous portion of the sauced beef onto the bottom buns, top with a heap of coleslaw, and cover with the bun tops. Serve immediately.