Bring the milk to a boiling point.
Melt the butter in a pot.
When it starts to crackle, add the flour at once.
Mix well with a wooden spatula or a whisk.
Cook the white roux for a few minutes. Beware not to let it brown. It must not get any coloration.
Pour the boiling milk on the cold roux.
Whisk and bring again to a boiling point. The bechamel will thicken.
the egg.
Add the yolk, keep the white aside for another use.
Laisser cuire quelques minutes en remuant constamment. Ajouter le gruyère rapé.
Transfer to a container or bowl and let cool. Brush the surface with a knob of butter to prevent a crust from forming.