Potato, Leek, and Mushroom Gratin
Notes: Using a mandoline to slice the potatoes makes a quick job of it. You can make this gratin up to 1 day ahead; bake it covered, let it cool, then chill. The next day, let gratin stand at room temperature for about 1 hour, then bake, uncovered, in a 350° oven until hot in the center, about 1 hour. If it starts to get too brown, cover loosely with foil. Prep and Cook Time: about 2 1/2 hours.