When you're ready to make the sauce, put the milk in a saucepan, then simply add the flour and butter and bring everything gradually up to simmering point, whisking continuously with a balloon whisk, until the sauce has thickened and becomes smooth and glossy.
Then turn the heat down to its lowest possible setting and let the sauce cook very gently forminutes, stirring from time to time. Meanwhile, taste and add seasoning.
