Sweat the onion cubes in the hot clarified butter until light yellow, add the sugar and let it caramelize. Add the potato cubes and sauté for 2 minutes. Now add the sauerkraut and steam it too.
Pour in the vegetable stock, add the spices and simmer gently for 25 minutes. Go in with the hand blender 3 or 4 times and chop the whole thing up a bit, but do not puree! Now add the leek rings and simmer for another 5 minutes.
Stir in the sour cream and, if necessary, add salt, pepper and sugar to taste. Scatter the chopped parsley on top and serve.