Preheat the oven to 400°F.

Combine the milk and cream with the leeks, garlic purée, rosemary, and nutmeg in a large heavy saucepan over medium heat. Add salt and white pepper and bring to a simmer. Whisk in 3 tablespoons of the butter, remove from the pan from the heat, and keep warm.

Using the remaining butter, generously coat the interior of an 11-by-7-by-2- inch baking pan. Set aside.

Season the potatoes with salt and pepper. Using about half of the potatoes, cover the bottom of the prepared pan with a layer of slightly overlapping potato slices. Pour half of the milk mixture over the potatoes. Make another layer with the remaining potatoes and cover it with the remaining milk mixture.

Bake for about 1 hour, or until the tip of a small, sharp knife is easily inserted into the center.

Remove from the heat and use as a side dish with meat, game, or poultry.