Begin by cutting the beef chuck roast into three equal chunks, about one pound each. Sear in a skillet over medium heat for 3-4 minutes on each side until browned.

Slice the onions and spread them evenly at the bottom of your slow cooker.

Place the seared beef chunks on top of the sliced onions in the slow cooker.

In a bowl, combine tomato paste, Dijon mustard, apple cider vinegar, and water. Add garlic powder, oregano, chili powder, smoked paprika, salt, and pepper, and stir until smooth.

Pour the sauce over the beef and onions in the slow cooker, ensuring the beef is well coated.

Cover the slow cooker and set it to LOW for 8 to 10 hours.

Once cooked, transfer the beef to a baking dish and shred it using two forks. Return it to the slow cooker and stir with sauce and onions.

Prepare coleslaw by mixing coleslaw mix with a dressing of mayonnaise, apple cider vinegar, garlic powder, salt, and pepper.

Serve the BBQ beef alongside the fresh coleslaw and any desired sides.