Wrap each potato in aluminum foil. Bake until tender, about 1 hour. Let cool. Cut each potato in half lengthwise. Scoop out each potato halves, taking care to keep a thin layer of flesh without damaging the skin. In a bowl, mash the potatoes with a potato masher. Refrigerate. Add the egg yolks, one at a time, to the mashed potatoes. Add the butter, cream, 375 ml (1 ½ cups) of cheese, and the nutmeg. Season with salt and pepper. In another bowl with an electric mixer, beat the egg whites until stiff peaks form.
