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.
1/4 cup (1/8 lb.) butter
1 pound fresh shiitake mushrooms, rinsed, stems removed, and sliced
1 tablespoon minced garlic
4 leeks (3 1/2 lb. total)
1 tablespoon minced fresh thyme leaves
3 3/4 pounds russet potatoes
1 3/4 cups low-sodium chicken broth
1 3/4 cups whipping cream
1/4 cup dry white wine
1 teaspoon salt
1/2 teaspoon pepper
3 1/2 cups shredded Gruyère cheese (12 oz.)
How to Make It
In a 12-inch frying pan or 5- to 6-quart pan over medium-high heat, melt 2 tablespoons of the butter. Add mushrooms and stir often until almost tender, 4 to 5 minutes. Add garlic and stir until mushrooms are golden and tender to bite, 2 to 3 minutes longer. Transfer to a bowl.
Trim and discard root ends and tough green tops from leeks. Cut stalks in half lengthwise and rinse well under running water, flipping layers to release dirt. Cut crosswise into 1/4-inch slices; you should have about 4 cups.
In the frying pan (no need to wash), over medium-high heat, melt remaining 2 tablespoons butter. Add leeks and thyme and stir often until leeks are limp and beginning to brown, 8 to 10 minutes. Stir into mushrooms.
Preheat oven to 350°. Peel and slice potatoes crosswise as thinly as possible (see notes). In a bowl, whisk together broth, cream, wine, salt, and pepper. In a buttered 9- by 13-inch baking dish (3-qt. capacity), evenly layer a third of the potatoes, half of the mushroom-leek mixture, and 1 cup cheese. Stir broth-cream mixture and pour a third evenly over the top. Repeat. Layer remaining potatoes, cheese, then cream mixture on top. Set dish in a 10- by 15-inch baking pan.
Bake until potatoes are tender when pierced and sauce is bubbly, 1 1/2 to 1 3/4 hours. After about 40 minutes, check browning; if top begins to get dark before potatoes are tender, cover loosely with foil. Let stand about 15 minutes before serving.