01 - Arrange sliced onions and minced garlic at the bottom of the slow cooker. In a bowl, whisk together beef broth, barbecue sauce, tomato paste, brown sugar, Worcestershire sauce, smoked paprika, cumin, salt, black pepper, and chili powder. Place the beef roast atop the onions, pour sauce over, ensuring even coating. Cover and cook on low for 8 hours until meat shreds easily.
02 - Remove beef from slow cooker, shred using two forks discarding excess fat. Skim fat from remaining sauce, return shredded beef to sauce, and stir thoroughly to coat.
03 - Whisk mayonnaise, apple cider vinegar, sugar, salt, black pepper, and celery seed in a large bowl. Add shredded cabbage and carrots, toss until evenly coated. Refrigerate until serving.
04 - Optional: toast buns lightly. Layer pulled beef generously on each bun, drizzle with additional barbecue sauce if desired, then top with a scoop of coleslaw. Serve promptly.