First drain the beans and collect the bean water, set aside. Heat the butter and stir in the flour. Then pour the milk and the bean water into the roux and bring to the boil until creamy. Season with salt, pepper and nutmeg and bring everything to a boil. Now add the beans and bring to the boil again. Finally, season again with salt, pepper and nutmeg. This is a great vegetable accompaniment to a wide variety of dishes, e.g., Schnitzel.