Melt the butter in a saucepan.
Stir in the flour and cook for 1-2 minutes.
Take the pan off the heat and gradually stir in the milk to get a smooth sauce. Return to the heat and, stirring all the time, bring to the boil.
Simmer gently for 8-10 minutes and season with salt and white pepper.
Stir in cheese and allow to melt. Don t re-boil the sauce or it will become stringy.