How to Make It
Preheat the oven to 350°. In a medium bowl, gently toss the potatoes with the leeks, olive oil, 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Transfer the potatoes to a 9-by-13-inch glass or ceramic baking dish and pat into an even layer. Cover the potatoes with foil and bake for about 1 1/2 hours, or until the potatoes are tender. Let cool completely, then cover with plastic wrap and refrigerate until chilled.
Preheat the oven to 450°. Cut the gratin into 8 squares. using a metal spatula, transfer the squares to a rimmed nonstick baking sheet. Bake for 30 minutes, or until the tops are brown. Serve immediately.