Melt the butter in a saucepan.
Stir in the flour to form a smooth paste.
Cook gently for about 1 minute or until the mixture is pale brown.
Remove from the heat and gradually stir in the milk or milk and stock.
Return to the heat and bring to the boil, stirring constantly.
Cook for about 3 minutes or until the sauce thickens.
Season to taste with salt and pepper.