Clean, wash and cut the leek into rings, put some aside.
Heat the butter, sauté the leek, dust the flour over it, pour in the stock, bring to the boil and cook for 10 minutes on a low heat.
Stir in the creme fraiche and mustard. Season to taste with salt, pepper and sugar. Garnish the soup with mustard seeds, parsley and leek rings cooked in broth.