Fish Filets in Creamed Sorrel Sauce

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 thei