Beef kidney by John Spottiswood.

Beef kidneys are tougher than lamb or veal kidneys, so they need to be cooked slowly using moist heat, say by braising them or adding them to a stew. 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.