Peal the beetroots and cut them into thin strips. Slice and chop the cabbage. Rinse and drain it.
Wash the leeks several times. Cut them into fine strips and leave to drain.
Peel and finely chop the onion and put it into a saucepan with the butter and heat for 5 minutes, stiring with a wooden spoon. When it takes on a caramel colour add the beetroot, the leeks and the strips of cabbage. Sprinke on the sugar and add the vinegar.
Allow to simmer for 3 minutes stirring constantly.
Add the stock cubes and 2.5 liters of water. Salt and pepper and cook over a low heat for 1 h.
Just before serving sprinkle with chopped parley and fennel.
Mix the fresh cream with the yogurt and serve on the side.
