Melt the butter in a saucepan, remove from the heat, add the flour and stir to a paste. Now add a little milk and stir to a paste, repeat the procedure.
The paste will turn into a cream and then into a thin liquid. Once you have reached the thin liquid stage, you can add the rest of the milk, the parsley, salt and a good grind of pepper.
Bring to the boil, reduce the heat to low and, stirring constantly, cook for a further 5-6 minutes or until the sauce starts to thicken.