This is a quick and easy basic white sauce

20 g | ¾ oz butter
20 g | ¾ oz plain flour
a pinch of dry English mustard
290 ml | ½ pint creamy milk
salt and freshly ground white pepper

1. Melt the butter in a heavy saucepan. Remove from heat.
2. Add the flour and the mustard and stir over the heat for 1 minute. Remove the pan from the heat, gradually stir in the milk and mix well.
3. Return the sauce to the heat and stir continuously until boiling.
4. Simmer for 2 minutes and season 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]