Melt the butter or margarine in a large saucepan and fry (saute) the onion
until soft but not brown. Add the cucumber and stock, bring to the boil
and simmer for 20 minutes. Beat together the egg yolk, 15 ml/1 tbsp of the
yoghurt and the cornflour until smooth. Add the hot soup, then transfer to
a food processor or blender and puree until smooth and creamy. Allow to
cool. Pour into individual dishes and garnish with the remaining yoghurt
and the chopped mint. Chill before serving.