Lombarde Sauce

Have in a saucepan half a bottleful of white wine, add to it two ounces of butter, one teaspoonful of chopped parsley, two finely chopped and blanched shallots and a quarter of a pound of chopped mushrooms. Let all simmer till the liquid is reduced, then add a pint of bechamel (No. 409), and a gill of cream, also the juice of a lemon and a pinch of mignonette.