Mushroom Gruyère Quiche
2 tablespoons olive oil, divided
1 large leek, white and light green thinly sliced and washed well to remove grit
1 small–medium yellow onion, diced
1¼ cups all-purpose flour
1 tablespoon plus 2 teaspoons cornstarch
¼ teaspoon salt
6 tablespoons butter, cold and diced
4 eggs, divided
6 ounces cremini mushrooms, sliced
1 tablespoon sherry cooking wine or port
½ cup plus 1 tablespoon heavy cream
1 cup plus 2 tablespoons sour cream
¾ cup coarsely grated Gruyère cheese
Heat 1 tablespoon olive oil in a large skillet over low heat. Add the leek and onion; sauté 30–40 minutes, stirring occasionally, until caramelized. Adjust heat so that leeks and onions are just beginning to brown after the first 10 minutes. Once caramelized, transfer to a bowl and set aside.
Meanwhile, in the bowl of a food processor or a large bowl, combine flour, cornstarch, and salt. Pulse or whisk to combine. Add butter and pulse or use 2 knives to cut the butter into the flour mixture until the butter is in pieces the size of rolled oats. Add 1 egg and pulse or mix (a fork works well) until a dough forms. Transfer the dough to a lightly floured surface and roll out into a 12-inch circle.
Arrange the dough in a 9-inch pie plate and press out any air bubbles. Crimp the edges and refrigerate, uncovered, for 30 minutes.
When the leeks and onions are caramelized, use the same skillet to heat the remaining tablespoon olive oil over low heat. Add the mushrooms and sherry cooking wine and sauté, stirring occasionally, for 8 minutes, until mushrooms are tender, brown, and the liquid has evaporated. Remove from heat and add to leek and onion mixture; stir to combine.
Preheat oven to 350°.
In a large bowl, whisk the heavy cream, sour cream, and remaining 3 eggs until smooth. Whisk in Gruyère.
Remove the quiche shell from the refrigerator and spread the leek and mushroom mixture in an even layer. Whisk the egg and cheese mixture once more to combine then pour over vegetables.
Bake until puffed, golden, and a toothpick inserted in the center comes out clean, approximately 35–50 minutes. Remove from oven and cool slightly on cooling rack before serving warm or room temperature.