Melt butter in a saucepan over medium heat.
Add the sifted flour and mix with a whisk or magic whisk until you have a smooth "roux" paste. Do not let it brown.
Remove from heat and pour in a little cold milk, stirring well to smooth the mixture.
Note: If you still have lumps, strain the paste through a fine sieve before adding the remaining milk.
Return the saucepan to the heat and pour in the remaining milk while continuing to stir.
Thicken while stirring, making sure to go well into the bottom and along the edges of the saucepan to prevent the sauce from sticking and burning.
Season with salt, a pinch of pepper and grated nutmeg.
The bechamel will thicken gradually. Remove from heat when it coats the spoon well.
Note: You can leave it more liquid by removing it from the heat earlier, or thicken it further depending on the recipe you're going to use it in. For a more liquid texture, you can add a little more milk. Bear in mind that it will thicken as it cools.
Use it immediately or store it for later by covering it with plastic wrap to prevent a skin from forming on top.