Peel the potatoes and slice them about ¼" (0.5cm) thick. Soak them in cold water for five minutes to remove any excess starch, them pat them dry using absorbent kitchen paper.
Butter a large, shallow ovenproof dish then melt the remaining butter in a small pan. Spread a layer of potato slices on the bottom of the dish. Drizzle a little butter over the top and season lightly with salt and pepper. Repeat these layers finishing with butter.
Cover the dish and cook in the middle of a preheated moderately hot oven (Mk 6 - 475ºF - 200ºC) for one hour. Sprinkle with chopped parsley and serve from the dish.