Place oxtail, onions, carrots, cabbage, celery, barley, onion soup mix, dill, cinnamon, bay leaf, and water in a large slow cooker. Cook on low until meat is tender, about 5 hours.
Remove meat and bones from slow cooker; cool on a plate until easily handled, about 10 minutes. Cut meat into small pieces; return meat to the soup and discard bones. Skim off any fat from soup; remove bay leaf.
Stir beets and beet greens into soup; cook on low until beets are tender, about 1 hour. Season with salt and pepper; top each bowl with about 1 tablespoon sour cream to serve.
