Wash potatoes well. No need to peel them, unless desired. Slice thin and drop into a pot of cold, salted water. Place pot over high heat and bring to a boil. Boil potatoes for 1 minute, drain, and rinse with cold water. Drain again and pat dry.
In a mixing bowl, combine ricotta and parsley, and season with salt, pepper and nutmeg.
In another bowl, beat the egg slightly and add enough heavy cream to make 1 cup of liquid.
Preheat oven to 350°F.
Lightly butter a shallow, oval gratin dish (approx. 9 x 12 inches).
Arrange a layer of one-fourth of the potato slices in the dish, slightly overlapping them. 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 slightly with a fork, if necessary, to allow the cream to spread evenly among them.
Bake on the center rack of 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.