POTATO-MUSHROOM GRATIN

POTATO-MUSHROOM GRATIN image

Yield 8

Number Of Ingredients 9

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5 tablespoons olive oil, divided
2 1/2 pounds medium Yukon Gold potatoes, peeled, cut lengthwise into 1/8-inch-thick slices
1 1/2 teaspoons coarse kosher salt plus additional for mushrooms
3/4 teaspoon freshly ground black pepper plus additional for mushrooms
1 1/4 cups (or more) heavy whipping cream, divided
1 cup freshly grated Parmesan cheese (about 3 ounces), divided
2 tablespoons chopped fresh thyme
6 garlic cloves, thinly sliced
12 ounces fresh crimini (baby bella) mushrooms, sliced

Steps:

Preheat oven to 375°F. Brush 13x9x2-inch glass or oval ceramic baking dish with 2 tablespoons oil. Arrange 1/3 of potatoes, slightly overlapping, in dish. Sprinkle with 1/2 teaspoon coarse salt and 1/4 teaspoon pepper. Pour 1/3 cup cream over; sprinkle with 1/4 cup cheese. Repeat layering 1/3 of potatoes, 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/3 cup cream, and 1/4 cup cheese 2 more times. Bake uncovered until potatoes are tender, adding cream by tablespoonfuls if dry, about 45 minutes. Remove from oven; maintain oven temperature. Sprinkle thyme and garlic slices over gratin. Toss mushrooms in medium bowl with 3 tablespoons oil; sprinkle with coarse salt and pepper. Arrange mushroom slices atop gratin around edge of dish. Drizzle with 1/4 cup cream; sprinkle with 1/4 cup cheese. Continue to bake uncovered until mushrooms are tender and potato edges are golden brown, about 20 minutes longer. DO AHEAD Can be made 1 day ahead. Cool 1 hour in dish; cover and chill. Before serving, cut gratin into 10 pieces. Transfer to rimmed baking sheet. Cover with foil and rewarm in 350°F oven, 10 to 15 minutes.