Wash potatoes well and trim away any discolored spots or eyes.
(You may peel potatoes if you like.) Slice thin and drop into a pot of cold, heavily salted water. Set over high heat and bring to a boil. Boil potatoes for 1 minute, drain, and rinse with cold water. Drain again and pat dry.
Combine ricotta and parsley and season generously with salt, pepper and nutmeg.
Beat the egg briefly and add enough heavy cream to make 1 cup of liquid. Season with salt, pepper and nutmeg.
Lightly butter a shallow, oval gratin dish measuring 9 x 12 inches. Arrange a layer of slightly overlapping potato slices in the dish. Dot with about one third of the ricotta mixture; sprinkle with one third of the Gruyère. Repeat, using all of the ingredients and ending with a potato layer.
Gently pour the egg and cream mixture into the dish, lifting potato slices with a fork if necessary to allow the cream to spread evenly among them.
Bake on the center rack of a preheated 350°F oven for 35 to 45 minutes, or until potatoes are tender and the cheese is browned and bubbling. Let sit for about 10 minutes before serving.