2-3 pounds potatoes
2 tablespoons unsalted butter
1-2 leeks, white part only, halved lengthwise and cut into 1/4-inch-thick slices
1 1/2 cups heavy cream
1 cup whole milk
1/4 teaspoon freshly grated nutmeg
Coarse salt and freshly ground black pepper
1 1/2 cups Gruyere cheese, coarsely grated
Preheat oven to 400 degrees. Fill a large bowl with water. Peel potatoes and add them to water as you peel, to prevent discoloration.
Melt butter in a medium saucepan over medium heat. Add leeks and cook, stirring, until soft, 3 to 5 minutes. Add cream, milk, and nutmeg; cook until bubbles form around the edge. Remove from heat.
Meanwhile, slice potatoes 1/8-inch thick. Place in even layers in a large shallow baking dish; season with salt and pepper. Pour leek-cream mixture over potatoes; sprinkle with cheese.
Transfer to oven and bake until potatoes are fork-tender and top is brown and bubbling, about 55 minutes. Remove from oven and let cool about 10 minutes before serving.