Trim the kidney leaving it covered with a thin layer of its suet. Season and colour it quickly on all sides in a small cocotte with a little butter; cover and place in the oven to cook for 15 minutes. Serve as it is in its cocotte.
The kidney is completed at the table in the following manner: transfer the kidney to a hot plate, place the cocotte on a spirit lamp, deglaze with ½ dl (2 fl oz or ¼ U.S. cup) excellent brandy and reduce by half. Meanwhile, cut the kidney into very thin slices and cover these with an overturned plate.
To the reduced brandy add 1 tsp mustard, 30 g (1 oz) butter in small pieces, the juice of a quarter of lemon and a pinch of chopped parsley. Mix these ingredients well together with a fork then add the prepared slices of kidney along with any of the juices which have drained from it. Reheat together without boiling and serve on hot plates.