Mushroom and Potato Gratin

Mushroom and Potato Gratin MUSHROOM SAUTE: 1 tablespoon extra-virgin olive oil 1 tablespoon unsalted butter 1 pound mixed fresh mushrooms (shiitakes, cremini, white), washed, trimmed & sliced 1/4-inch thick (5 to 6 cups) 2 cloves garlic, minced 1/2 teaspoon kosher salt 2 tablespoons chopped fresh flat-leaf parsley freshly ground black pepper 2 to 3 tablespoons heavy whipping cream, broth or lemon juice additional chopped herbs (optional) GRATIN: 1 tablespoon unsalted butter, softened at room temperature 1 cup low-sodium chicken broth 1 cup heavy whipping cream 2 1/2 pounds russet potatoes 1 tablespoon chopped fresh thyme 2 1/2 teaspoons kosher salt freshly ground black pepper 1/2 cup (about 2 ounces) finely grated Gruyère cheese Prepare the mushrooms: Heat the oil and butter in a 12-inch skillet over medium heat until the butter foams. Add the mushrooms and the garlic. The mushrooms will begin to absorb the fat in the pan. Sprinkle with salt and stir with a wooden spoon until the mushrooms begin to release their moisture. Sauté until some of the moisture cooks out. Add pepper to taste. Deglaze the pan with heavy cream, broth or lemon juice. Add additional herbs if desired. Transfer the mushrooms to a bowl. Preheat the oven to 375°F. and butter a 9x13-inch baking dish with the softened butter. Heat a skillet to medium-high heat. Pour in the broth and scrape the pan with a wooden spoon until most of the cooked-on bits are released into the broth. Combine the broth with the cream in a measuring cup with a spout. Peel and slice the potatoes very thinly (about 1/8-inch). In a large bowl toss the sliced potatoes with the thyme, salt and several grinds of pepper. Overlap half of the potato slices in two even layers to cover the bottom of the buttered baking dish. Distribute the mushrooms over the potatoes. Cover the mushrooms with two more overlapping layers of the remaining potato slices. Pour the broth and cream mixture over the potatoes. Sprinkle the cheese evenly over the top. Cover the gratin with foil and bake it for 30 minutes. Uncover it and press the potatoes down with a spatula. Bake uncovered, rotating the pan occasionally for even browning, until the top is browned and the potatoes feel tender when poked with a fork, another 25 to 30 minutes. The cream will be gently bubbling. Let the gratin sit for about 15 minutes before serving.