Pre-heat the oven to 170°C/325°F/Gas mark 3. Slice the potatoes very thinly in a food processor or on a mandoline, then soak in cold water for 1 hour to remove the starch. Rinse under running cold water and dry carefully. Use a little of the butter for greasing a gratin dish. Layer the potatoes in it, sprinkling them with a little of the nutmeg, salt and pepper as you go. Pour in half the cream when the dish is half full, then continue to layer the rest of the potatoes, seasoning and cream until the dish is full. Dot with the remaining butter and bake for 2½ hours, or until the top has turned golden brown.