2 tablespoons of flour
½ litre of milk
¼ litre of water
1 egg yolk
100g whipped cream
the juice of one lemon
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 juice.