Arrange onion, sweet potatoes, parsnips and celery root in bottom of slow cooker. Season beef all over with salt and black pepper. Rub flour all over beef. Place roast on top of vegetables in slow cooker.
Whisk together tomatoes, maple syrup, chili powder, cumin, garlic powder, and onion powder. Pour mixture over beef.
Cover and cook on LOW for 12 hours or on HIGH for 8 hours.
Remove beef from slow cooker and shred with 2 forks. Serve 1/3 of shredded beef with this meal (and all of the vegetables) and refrigerate remaining beef for another use.