A classic gratin, with layers both crisp and creamy, is a rich composite of sliced potatoes, grated Gruyere, heavy cream, and seasonings.
Preheat oven to 350 degrees. Butter a 9-inch square baking dish, and set aside.
Whisk together cream, 1 teaspoon salt, 1/4 teaspoon pepper, and the nutmeg in a small bowl, and set aside. Toss potatoes, 1 teaspoon salt, and 1/4 teaspoon pepper in a large bowl.
Arrange one-third of potatoes in buttered dish, overlapping slices. Sprinkle with one-third of the cheese. Repeat two more times with remaining potatoes and cheese (end with a cheese layer). Pour reserved cream mixture over top layer. Gently shake dish back and forth to distribute evenly.
Cover with foil; bake 30 minutes. Remove foil; bake until bubbling and well browned, about 30 minutes more. Let cool slightly before serving.