Melt a knob of butter
and pour in the liquid, made up of the milk and water, in which you
have mixed the flour. Let it cook slowly, always stirring, until it
becomes a smooth bechamel sauce. Season with salt.
Blend in the egg
yolk, mixing with a whisk, so that the egg doesn't have time to cook,
then, still mixing, add the rest of the butter in little pieces,
taking care that the pan stays hot, over a very low flame. The sauce
will whip into a cream.
Take it off the
heat, incorporate the whipped cream and at the very end, the lemon