Remove the skin and fat from the kidneys. Cut in half the long way and then cut in ⅛-inch-thick slices.
Heat butter and oil in a heavy skillet to very hot, but not smoking. Add the kidneys. Stir-cook for no longer than 5 minutes. They are done when they are hot. Sprinkle with salt and pepper.
Pour contents of pan into a strainer in the sink. Let kidneys drain. Never use juice from the kidneys. It has an ammonia odor. Kidneys are now ready to be added to a prepared sauce, to be flamed, or to be tossed in butter. They are not cooked again, only reheated.