Preheat oven to 350 degrees. Butter a 12-cup baking dish. Melt butter in a skillet over medium heat. Add leeks, and cook until translucent, 3 to 4 minutes.
Arrange 1/3 of the potatoes in dish, slightly overlapping slices. Sprinkle with 1/2 of the salt, 1/2 of the nutmeg, and pepper, followed by 1/2 of the leeks and 1/3 of the cheese. Repeat. Top with remaining potatoes in a spiral. Sprinkle with remaining cheese. Combine cream and stock. Pour over cheese and potatoes. Cover with parchment and foil. (Mixture can be refrigerated overnight.)
Bake for 30 minutes. Increase temperature to 425 degrees, uncover, and cook until top is golden brown and potatoes are tender, about 45 minutes. Let rest for 15 to 30 minutes before serving.