Trim the chops of excess fat. Skin the kidneys and cut into 1/2 cm rounds. Peel and slice the onions thinly. Peel and slice the potatoes to the thickness of a pound coin. Mix the onions with the mace, thyme and cayenne then layer the hotpot as follows, taking care to season each layer with salt and pepper: place a thin layer of potatoes on the bottom of the pot, cover with half the onion-and-herb mix, stand the chops on top and wedge more onions in between. Scatter the kidneys around, top with the rest of the onions and then put the remaining potatoes on top. Pour over the stock which should come halfway up the potatoes, dot the surface with butter and cover. Place in a slow oven (150C/300F/Gas 2) for three hours. Check once or twice to make sure the hotpot is not cooking too fast or drying up. Remove the cover and brown in a hot oven (at least 450F/230C/Gas 8) for 10-15 minutes. Serve at once.