1. Preheat the oven to 350°. Using a pastry brush, coat the bottom and sides of a 4-to 5-quart casserole at least 6 inches deep with the 1 tablespoon of softened butter.
2. Spread about one third of the potato slices evenly over the bottom of the casserole, and place 3 lamb chops side by side on top. Sprinkle the chops with ½ teaspoon of the salt and a few grindings of pepper.
3. Add half the kidneys, 3 oysters, half the mushrooms and half the onions, and cover with another third of the potatoes. Place the 3 remaining chops on the potatoes and sprinkle them with the remaining salt and a few grindings of pepper.
4. Add the remaining kidneys, oysters, mushrooms and onions, and spread the rest of the potatoes evenly over the top. Pour in the water. Dot with the bits of butter.
5. Cover and bake in the middle of the oven for 1½ hours. Remove the cover and bake for 30 minutes longer, or until the top is brown. Sprinkle with parsley and serve directly from the casserole.
