Remove the outer leaves and the end of the root from the leek, then cut the stalks into rings, wash thoroughly, drain.
Meanwhile, heat water in a saucepan, add salt and add the washed leek rings to the boiling salted water and cook in it for about 20 minutes. Drain the leek in a kitchen sieve and collect the cooking water in a mug. Let the leek stock cool down to prevent the bechamel sauce from clumping.
Heat the butter in the saucepan, add the flour and sweat a little. Deglaze with 250 ml of leek stock and stir well with a whisk so that the sauce is creamy and does not clump, bring to the boil so that the flour taste disappears. Then add the cream and milk, season with boiled broth and freshly grated nutmeg and heat again briefly and then add the cooked leek to the bechamel sauce and stir carefully so that the leek is well distributed in the sauce.