This sauce is especially good for boiled fish, and the quantities indicated below are sufficient for a piece of fish or a whole fish weighing about a pound.
Put on the fire in a little saucepan the flour and butter, and when the flour begins to be browned, pour over it little by little the broth of the fish, that is to say of the water in which the fish has been boiled. When you see that the flour does not rise in the boiling water, take away the sauce from the flour and pour over the olive oil and the yolk, stirring and mixing everything well. Squeeze the lemon in the sauce and season generously with salt and pepper. Let it cool and then pour over the fish that is to be served with a sprig of parsley.
This sauce must have the appearance of a cream and must not be too liquid, in order that it may remain attached to the fish.