1. Heat the stock. Melt the butter in a large saucepan, stir in the flour and cook over a low heat for 2 minutes.
2. Remove from the heat and pour in the stock all at once, beating energetically with a balloon whisk to prevent lumps forming.
3. Return to the heat, add the Sauerkraut and the cumin, cover and leave to simmer gently for about 30 minutes.
4. Add salt and pepper to taste and serve very hot.