- 4 tablespoons (½ stick) butter
- 1 pound mushrooms, halved if large
- 1 tablespoon minced garlic
- 4 cups thinly sliced leeks (white and pale green parts only; about 4 large)
- 1 tablespoon minced fresh thyme
- 1 ¾ cups canned low-salt chicken broth
- 1 ¾ cups whipping cream
- ¼ cup dry white wine
- 1 teaspoon salt
- ½ teaspoon ground black pepper
- 3 ¾ pounds russet potatoes, peeled, thinly sliced into rounds (about 8 cups)
- 2 ½ cups grated Gruyère cheese (about 10 ounces) Guoda works well too.
Melt 2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms and sauté until almost tender, about 5 minutes. Add garlic and sauté until mushrooms are tender and golden, about 3 minutes longer. Transfer mushroom mixture to bowl. Melt remaining 2 tablespoons butter in same skillet over medium high heat. Add leeks and thyme and sauté until leeks are tender and beginning to brown, about 8 minutes. Add leek mixture to bowl with mushrooms.
Preheat oven to 400˚F. Butter 13 x 9 x 2-inch baking dish. Whisk broth, cream, wine, 1 teaspoon salt and ½ teaspoon pepper in large bowl to blend. Layer ⅓ of potatoes on bottom of prepared dish. Top with half of mushroom-leek mixture, then ¾ cup cheese. Top with half of remaining potatoes. Pour half of cream mixture over. Top with remaining mushroom-leek mixture and ¾ cup cheese. Cover with remaining potatoes. Pour remaining cream mixture over; sprinkle with remaining cheese. Bake gratin uncovered until potatoes are tender and sauce bubbles thickly, about 1 hour 20 minutes. Let gratin stand 30 minutes before serving.