Clean the leek, leaving most of the green parts on. Slit lengthways, wash thoroughly and cut into fine rings. Set aside 2 tablespoons of the dark leek rings. Sauté the rest of the leek in butter or margarine, pour in the stock and simmer covered for 10 minutes. Puree finely. Bring the cream to the boil again while stirring vigorously and season to taste with horseradish and salt. Bring the dark leek rings to the boil briefly in the soup. Arrange the soup in cups immediately and serve.