Pork kidneys by John Spottiswood.

Like beef kidneys, these are too tough to cook using dry heat. It's best to cook them slowly using moist heat. To prepare them, first cut off the outer membrane, then cut them lengthwise to expose a white blob of fat in the middle, which should be discarded. Next, soak the kidney in acidulated water or buttermilk for about an hour. This will make the flavor much more mild.