Béchamel sauce

Preparation

Melt the butter in a deep saucepan, mix in the flour, and stir until it cooks without taking on color.
Add the hot milk and beat the mixture with a whisk until it begins to boil.
Loosen heat to a minimum and slowly cook for about 30 minutes without stirring, defoaming the surface occasionally.
Season with salt, pepper and nutmeg.

Variations:
You can add chopped onion and sauté it in butter before baking the flour.
You can also add a little tomato along with the onion.