20 g | ¾ oz butter
20 g | ¾ oz plain flour
pinch of dry English mustard
a pinch of cayenne pepper
290 ml | ½ pint milk
55 g | 2 oz Gruyere or strong cheddar cheese, finely grated
15 g | ½ oz parmesan cheese, freshly grated (optional)
salt and freshly ground black pepper

1. Melt the butter in a heavy saucepan and stir in the flour, mustard and cayenne pepper. Cook, stirring for 1 minute.
2. Remove the pan to the heat and stir until boiling. Simmer, stirring well, for 2 minutes. Remove from heat.
3. Add all the cheese, and mix it well, but do not reboil.
4. Season to taste with salt and pepper.

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Excerpt from Leiths Cookery Bible

[Leith, P., and Waldegrave, C., (2003), Leiths Cookery Bible, 3rd edition, Bloomburys, UK]