Slice the potatoes into 1/8-inch slices on a mandoline. Reserve in a bowl of water until ready to use. In a medium bowl, mix together the Gruyere, Parmesan and Pecorino and set aside.
In a small pot, melt the butter with the rosemary, garlic and nutmeg. Mix 1 tablespoon of the butter mixture with the breadcrumbs in a small bowl and set aside.
Grease the interior of a 10-inch saute pan or baking dish with some of the butter mixture.
Drain the potatoes, then dry on towels.
Arrange a single layer of potatoes in the pan in a circular motion, followed by some of the onions, then approximately 2 tablespoons of the butter mixture, then a sprinkle of the cheese mixture. Season with salt and pepper. Continue to repeat in single layers with the remaining ingredients, for about 4 layers total. Drizzle with the heavy cream and top with the breadcrumb mixture.
Bake until the top is golden brown and the potatoes are very tender, 1 hour 30 minutes to 1 hour 45 minutes.