Sweat the onions and celery in butter or margarine. Chop the kale and add it. Add the potatoes, broth, nutmeg and mustard seeds and cook for 20 minutes. Finally add some heavy cream or soy cream and walnut oil.
In the meantime, toast the whole grain croutons in a pan with a little butter or margarine.
Puree the soup finely and serve with roasted wholemeal croutons and a little whipped cream or soy cream.