Chop the onions, cook to a golden colour in a little butter, and sprinkle with flour. Add these to the stock and bring to the boil. Blanch the beef, cut in small pieces, and blanch the cabbage, coarsely chopped.
Drain and add to the soup and season with salt, pepper and fennel. Simmer very gently for 1½ hours. Sour cream can be stirred into the soup a few minutes before serving, or can be served separately.