Rinse the mushrooms with some of the lemon juice and stem them. Dry and quarter them and place in a bowl with more lemon juice so they remain white. In a large frying pan, sear the kidneys over a high heat. Remove them and reserve. They will exude a lot of juice. In the same pan, place the butter and the ham and cook over a moderate heat. Add the flour and brown it. Add the Madeira and the white wine and bring to a boil (the result will be pasty looking), add the mushrooms and cover. Cook and stir while they lose their water. Pour over the kidneys. Stir well, and if necessary add a tablespoon or two of water so the juices nearly cover the kidneys. Bring to simmering point slowly and simmer for 10 minutes. Serve immediately, the kidneys must remain pink inside. Accompany with mashed potatoes.
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2 beef kidneys, fat removed, cut into walnut-sized pieces
