Preheat the oven to 220 C, 200 Fan, Gas 7. Brown the chops and the kidneys in half the butter or dripping over a high heat to give them a little colour.
Layer the potatoes, chops, kidneys, onions and mushrooms in a deep casserole, seasoning well between each layer. End with a layer of potatoes, neatly overlapping and covering the contents of the dish.
Pour over the stock- there should be enough to come about half way up the ingredients so add a little water if needed. Brush the remaining dripping or butter over the top layer of potatoes then season well.
Cover the casserole and place in the oven . Cook at this temperature for 20-25 minutes then reduce the tempurature to 150 C, 140 Fan, Gas 2 and cook for a further 2 hours.
Finally remove the lid, increase the oven temperature back to 220 c, 200 Fan, Gas 70 and cook for a final 20-30 minutes until the top layer of potatoes is browned.