Heat 50 g (2 oz) of butter in a pan, add 100 g (3½ oz) of finely chopped onion and 1½ tbs finely chopped shallot and cook without colour. Moisten with 3 dl (½ pt or 1¼ U.S. cups) vinegar and reduce by half.
Add 5 dl (18 fl oz or 2¼ U.S. cups) of the Court-bouillon from the fish under preparation and thicken with 45 g (1½ oz) Brown Roux or 50 g (2 oz) Beurre Manié. Allow to simmer gently for 8 to 10 minutes.
Finish the sauce by adding 1 tbs chopped Fines Herbes, 1½ tbs small capers, 1½ tbs dry Duxelles and ½ tbs of anchovy essence and 60 g (2 oz) butter, or 80–100 g (3– 3½ oz) ordinary anchovy butter.
This sauce is specially suitable for boiled fish of second quality, e.g. skate.