There's nothing simple about it!

I always get this sauce wrong - the secret is to watch it like a manic obsessive hawk and not let it overcook, which I invariably do.

What you need is equal amounts of butter, flour and milk. Melt the butter in a pan over a very small flame (or even better, in a bain-marie). When it's nice and clear and beginning to simmer slightly, whisk in the flour and keep whisking until the mixture is smooth and even.

Now add the milk, and keep your eyes peeled! The damn infuriating thing will be ready just when you least expect it. It should be bubbling, very white and very creamy - with no visible traces of yellow fat. Speed it off the flame and use it as soon as possible - under no circumstances should it be reheated, it'll be awful.

This is the best and easiest way of making what is also known as "white sauce" for lasagne and other bakes.