* Place butter in a saucepan and turn the heat to low. When the butter is melted, remove the saucepan from the stove.
* Sift the flour into the butter a little at a time stirring continuously to combine them.
* Add the milk a little at a time.
* Put the saucepan back on the stove and slowly stir with a wooden spoon, until the sauce starts boiling and becomes thicker.
* Turn heat off. Add salt, and stir in the grated parmigiano cheese and pinch of nutmeg.