Heat 50 g (2 oz) butter in a pan; add 100 g (3½ oz) ordinary Mirepoix cut into Brunoise and cook until lightly coloured. Moisten with 2 dl (7 fl oz or ⅞ U.S. cup) white wine and 3 dl (½ pt or 1¼ U.S. cups) Fish Stock and allow to cook gently for 25 minutes.
Pass through a fine strainer, allow to stand for a few minutes and remove the fat. Reduce it to ½ dl (2 fl oz or ¼ U.S. cup), add 1 tbs Madeira and thicken this reduction with 2 egg yolks; finish with 100 g (3½ oz) butter, 30 g (1 oz) Beurre Rouge and a few drops of anchovy essence.
This sauce is specially suitable for serving with boiled or poached fish.