In a saucepan, melt the butter over low heat.
Add the flour. Mix quickly with a whisk. Stir until the mixture is smooth and without lumps.
Heat the milk, without bringing to a boil. Pour the milk slowly over the roux, while stirring vigorously with a whisk to prevent the formation of lumps. Keep mixing constantly, until you have a fairly thick consistency (about 10/15 minutes).
Add a pinch of fine salt and grated nutmeg to taste. For an even richer flavor, add a little Parmigiano reggiano cheese (optional).
