If there is one ingredient in particular that signifies fall to me, it’s mushrooms. I love them year round, but come October, I start to use more of them in my cooking than usual, especially the dried kind. And no dried mushroom is arguably more flavorful and more prized than porcini mushrooms. Layered between thin slices of potatoes and garlic-flecked cheese, this gratin makes for a decadent side dish to any cold-weather meal.
2 ounces dried porcini mushrooms
1 cup boiling water
1 tablespoon butter
1 tablespoon olive oil
1/4 cup plus 2 tablespoons grated Parmesan cheese
1 cup mascarpone cheese
3/4 cup whipping cream
3 garlic cloves, chopped
Pinch of grated nutmeg
2 1/2 pounds russet potatoes (about 5), peeled, cut crosswise into 1/8-inch-thick slices
1. Place porcini and boiling water in medium bowl. Place small plate atop bowl to keep covered and let soak 20 minutes. Drain and coarsely chop mushrooms.
2. Melt butter with oil in medium skillet over medium heat. Add mushrooms and sauté until beginning to brown, about 3 minutes. Sprinkle with salt and pepper and remove from heat. Whisk 1/4 cup Parmesan cheese, mascarpone cheese, whipping cream, garlic and nutmeg in a bowl; season with salt and pepper.
3. Preheat oven to 325°F. Butter wide shallow 2-quart baking dish. Arrange 1/4 of potato slices in bottom of dish. Sprinkle lightly with salt and pepper. Scatter 1/4 of mushrooms over. Repeat. Spread half of cheese mixture over, shaking dish to settle. Repeat with remaining potatoes and mushrooms in 2 layers each; spread remaining cheese mixture over. Sprinkle 2 tablespoons Parmesan over. Place gratin dish on rimmed baking sheet.
4. Bake gratin until top is brown and sauce is bubbling at edges, about 1 hour 15 minutes. Let gratin rest 15 minutes before serving.