Place 200 g (7 oz) sliced onions in a pan with 6 dl (1 pt or 2⅝ U.S. cup) milk, a pinch of salt, pepper and grated nutmeg and bring to the boil. Allow to cook gently until tender; drain well, reserve the milk, then chop the onions.
Prepare a White Roux with 40 g (1½ oz) butter and 40 g (1½ oz) flour, mix in the milk from the onions, bring to the boil and add the chopped onions, keeping the sauce very thick. Allow to simmer very gently for 7–8 minutes.
This sauce is a useful accompaniment with rabbit, poultry, tripe, boiled mutton, braised game etc. and is always poured over the meat with which it is served.