Vegetarians may like to know you can buy a vegetarian style Parmesan from Bookham and Harrison Farms Ltd.
The Delia Online Cookery School: Watch how easy it is to make the sauce, just click the image to play.
This recipe is from Delia's Complete How to Cook.
When you're ready to make the sauce, put the milk in a saucepan.
Then simply add the sauce flour, butter and seasoning and bring everything gradually up to simmering point over a medium heat, 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 for 5 minutes to take the raw taste out of the flour, stirring from time to time. After that stir in the cheddar and grated parmesan.
Finally taste and check for seasoning.