Soak the kidney in tepid salt water for 15 min. Wash well, skin if necessary, remove the core and any fat and cut the kidney into slices about ¼ in. thick. Cut the bacon into pieces and the onions, tomatoes and mushrooms into slices. Put alternate slices of kidney, bacon, onion, tomatoes and mushrooms in a casserole, seasoning each layer. Three-quarters fill the casserole with stock and cover the top with a thick layer of sliced potatoes. Place some bacon rinds on top. Cover and cook in a moderate oven (350° F., Gas 4) for 2½ hr. Remove the lid and bacon rinds 1 hr. before serving. Dot the top with small pieces of bacon fat and allow the potatoes to brown. Sprinkle with finely-chopped parsley.