Clean the leek and cut into rings as thick as a finger. Put in a saucepan and cover completely with water. Season with salt and sugar, add a little margarine. Cook everything for 15 minutes.
Drain the leek and collect the water. Melt the butter in the pan and use the flour to make a light roux. Now pour in the leek water until a creamy sauce is formed. Season the sauce with nutmeg (possibly with salt).
Add the leek to the sauce and stir. Boiled potatoes or jacket potatoes go well with this.