Melt butter in a saucepan. Stir in an equal amount of flour and cook the mixture for just under a minute - known as a roux.
Stir in milk, a little at a time, making sure to stir well so that no lumps form.Bring the mixture to the boil, stirring constantly, so that the mixture thickens and becomes glossy. Boil for a couple of mins.
To make a cheese sauce, add grated cheese off the heat. Season well.