Heat 75 g (2½ oz) butter in a pan, add 300 g (11 oz) finely chopped onion and cook without colour. Moisten with 4 dl (14 fl oz or 1¾ U.S. cups) white wine and reduce by two-thirds. Add 6 dl (1 pt or 2⅝ U.S. cups) Sauce Demi-glace and simmer gently for 10 minutes.
Pass the sauce through a fine strainer and finish away from the heat with a pinch of sugar and 2 tsp of English mustard diluted with a little water.
This sauce is usually served to accompany grilled pork.