From:
The Thorough Good Cook
Sauces: 51. Richelieu
Chop up very fine six ounces or so of
mushrooms lightly fried in
butter, and one or two
truffles peeled. Put into a
saucepan two small spoonfuls of smooth sauce;
reduce it slightly, then take it off the fire and mix in the mushrooms and the truffles; add three yolks of eggs, and boil the
sauce for some two or three minutes, stirring it all the time. It is best to make this sauce in a shallow saucepan and to pour it afterwards into a smaller one. Just before serving mix in a piece of
butter, but do not let it boil again. This sauce should be
thick; but if it be too thick, thin it with the juice of mushrooms or with
broth.