No fish sauce is subtler or more exciting than a creamed velouté-sorrel sauce—and it is not fragile; the novice need not be intimidated by a threat of collapse. The initial velouté should be very lightly bound with flour (most people overthicken sauces); subsequent reduction and the sorrel purée will give it needed body and the terminal incorporation of butter will suffuse it with velvet. When saucing poached filets, one must naturally profit from the enriching virtues of their poaching liquid to reduce it and add it to the sauce, but a simple velouté combined with a sorrel purée, creamed and buttered, remains an exquisite sauce and a perfect accompaniment to fish preparations poached in a bain-marie that offer no supplementary cooking liquid of their own.