Yukon Gold Potato and Wild Mushroom Gratin

Preheat oven to 375 °F. Melt butter in heavy large skillet over medium-high heat. Add mushrooms and sauté until brown and soft, about 6 minutes. Generously butter 11x7x2-inch baking dish. Peel and cut potatoes into 1/8-inch-thick slices. Arrange 2 layers of potatoes in prepared dish. Spoon 1/3 of mushrooms over potatoes. Sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Repeat 2 more times with potatoes, mushrooms, salt, and pepper. Pour cream over potatoes. Cover with foil and bake 45 minutes. (Can be made 2 hours ahead. Let stand at room temperature.) Uncover and continue baking until golden brown and set, about 20 minutes longer (30 minutes if room temperature). Let stand 10 minutes before serving.