Clean the leek and cut into rings. Heat the butter in a saucepan and sauté the leek rings in it. Dust the flour over it, pour in the stock, bring to the boil and cook for about 10 minutes on a low heat.
Stir in the crème fraiche and mustard and season with salt and sugar to taste. Serve garnished with some mustard seeds and parsley cooked in broth, if desired.