Melt the butter in a saucepan or saucepan at medium temperature.
Add the flour to the melted butter, reduce the heat to low and mix well until smooth.
Add ⅓ of the milk and mix thoroughly until smooth.
Add another ⅓ portion of milk and mix again.
Add the rest of the milk, salt, pepper, nutmeg and whisk the sauce until it thickens. You need to cook the sauce on low heat and constantly stir with a whisk so that the sauce does not burn and turns out to be homogeneous.
Once the sauce has thickened, remove it from the stove. The sauce is ready!