- 1/2 lb fresh wild or exotic mushrooms such as chanterelles or shiitakes (discard shiitake stems), trimmed and coarsely chopped
- 2 1/2 tablespoons unsalted butter
- 3/4 lb fresh cremini mushrooms, trimmed and sliced 1/4 inch thick
- 1 1/2 teaspoons minced garlic
- 3 lb russet (baking) potatoes
- 1 1/2 cups heavy cream
- 1 1/2 cups whole milk
- 1 1/2 teaspoons salt
- 1/2 teaspoon white pepper
- 1/4 teaspoon freshly grated nutmeg
- 2 oz finely grated Gruyère (1 cup)
- Special equipment: a 15- by 10- by 2-inch oval gratin dish or other 3-qt shallow baking dish
- Put oven rack in middle position and preheat to 400°F.
- Cook chanterelles or shiitakes with salt and pepper to taste in 1 tablespoon butter in a large nonstick skillet over moderate heat, stirring, until liquid mushrooms give off is evaporated and mushrooms are tender, about 8 minutes, then transfer to a bowl. Cook cremini in remaining 1 1/2 tablespoons butter in skillet, stirring, until liquid is evaporated and mushrooms are tender, about 8 minutes, then transfer to bowl with wild mushrooms. Toss mushrooms with 1 teaspoon garlic.
- Peel potatoes and cut crosswise into 1/8-inch-thick slices (preferably with an adjustable-blade slicer). Bring potatoes, cream, milk, salt, white pepper, nutmeg, and remaining 1/2 teaspoon garlic to a boil in a 4- to 6-quart heavy pot, stirring once or twice, then remove from heat.
- Transfer half of potatoes to buttered gratin dish with a slotted spoon, spreading evenly. Spread mushrooms evenly over potatoes, then top with remaining potatoes. Pour cooking liquid over potatoes and sprinkle with cheese.
- Bake gratin until top is golden brown and potatoes are tender, 45 to 55 minutes. Let stand 10 minutes before serving.