1 pound russet potatoes
1 pound sweet potatoes
2 large leeks
2 tablespoons olive oil
2 tablespoons butter, plus extra for greasing baking dish
1 1/2 cups heavy cream
2 large cloves garlic, split
Salt and pepper
3/4 cup grated Parmesan
Preheat the oven to 350 F.
Butter a glass or ceramic 2 quart baking dish.
Peel the potatoes and slice thinly, 1/8 inch, either by hand or with a mandoline. Place in a bowl. Wash the leeks, checking for grit or sand, and slice the whites and the pale green thinly.
Over medium heat, add olive oil, leeks and a pinch of salt and sauté, slowly until soft and caramelized, 7-10 minutes. Remove from heat.
In a pot, slowly bring the cream and garlic to a simmer with a pinch of salt and pepper. Allow to barely simmer for a few minutes, remove from heat and keep warm.
Make a layer of mixed sweet and russet potatoes in the prepared baking dish. Sprinkle the potatoes with salt and pepper, and scatter over some of the leeks. Repeat until the potatoes and leeks are used up – this is kind of free form, so don't worry too much about being exact. Pour the warm cream over all, followed by the Parmesan.
Cover the potatoes tightly with foil and bake 40 minutes. Remove cover and continue to bake an additional 20 minutes. Potatoes should be cooked through. Run under the broiler a couple of minutes if a browner top is desired. Let rest 10 minutes before serving.