PHOTO: Kelly Sue DeConnick/Flickr
Place carrots, onions and beets in large soup pot. Add enough boiling water to barely cover vegetables, and simmer gently, covered, for about 20 minutes. Add butter, stock, cabbage and vinegar and simmer 15 minutes more.
Combine sour cream and grated cucumber.
Place soup in serving bowls, and top each bowl with dollop of sour cream-cucumber mixture.