If you can find them, it is worth seeking out wild mushrooms for this gratin in which layers of potato are baked to a rich golden brown with mushrooms and cream. Heavy whipping cream makes this an extravagant party dish. For lighter, everyday fare, you can substitute half and half, whole milk, or chicken stock.

Steps

1. Preheat oven to 375 degrees. Butter a 9-by-13-inch baking dish.
2. Heat a wide skillet over medium heat. Add olive oil, mushrooms, and salt. Sauté and stir occasionally until mushrooms release their liquid and begin to brown. Add garlic and parsley and cook, stirring, 30 seconds more. Take off heat and set aside.
3. Pour cream into a big heavy pot. Use a mandoline to slice potatoes lengthwise into pieces ⅛-inch thick or even thinner. Immediately put potato slices in the cream-filled pot, to avoid discoloration. Add salt and stir to mix.
4. Bring potatoes to a boil over high heat, stirring constantly with a wooden spoon to prevent sticking. Turn down to a simmer. Continue to cook, stirring gently, for 5 minutes. The potatoes will absorb much of the cream.
5. Lift half of the potatoes out of the cream and into the baking dish. Use the back of a spoon to press down potatoes to form an even layer. Sprinkle lightly with salt. Add mushrooms and distribute evenly. Add remaining potatoes and press firmly into place. Pour over remaining cream. Press potatoes down one more time, until the cooking liquid rises to the top of the potatoes. Sprinkle potatoes with Parmesan and black pepper.
6. Cover baking dish with foil. Bake for 30 minutes. Uncover and bake until potatoes are tender and golden brown, about 15 more minutes. Let rest for 15 minutes before serving.