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.