Start by trimming excess fat from the beef chuck roast. This will help reduce the greasiness of your final dish.
In a medium bowl, combine the BBQ sauce ingredients: ketchup, water, white vinegar, brown sugar, honey, dry mustard, Worcestershire sauce, salt, black pepper, cayenne pepper, crushed red pepper flakes, and minced garlic. Stir until well combined.
Place the beef chuck roast in the slow cooker and pour the prepared BBQ sauce over the top, ensuring the beef is well-coated. Cover and cook on low for 8 hours or on high for 4 hours until the meat is tender and shreds easily with a fork.
While the beef is cooking, prepare the coleslaw. In a large mixing bowl, combine diced onion, coleslaw mix, Miracle Whip, vegetable oil, white sugar, white vinegar, salt, and poppy seeds. Mix thoroughly until the coleslaw is well coated. Refrigerate until ready to serve.
Once the beef is cooked, carefully remove it from the slow cooker and place it on a cutting board. Using two forks, shred the meat into bite-sized pieces. Return the shredded beef to the slow cooker and mix it with the remaining BBQ sauce.
Serve the Super Bowl BBQ Beef on soft buns or with a side of fluffy rice, topped with a generous scoop of coleslaw.