For the filling
2 tablespoons (28 g) unsalted butter
1 tablespoon (14 g) extra virgin olive oil
1 small yellow onion, peeled and diced
1 pound (16 ounces) white or baby portobello mushrooms, sliced*
1/2 teaspoon kosher salt
1/8 teaspoon freshly ground black pepper
4 eggs (200 g, weighed out of shell)
1 1/3 cups (10 2/3 fluid ounces) half and half, or 1 cup (8 fluid ounces) whole milk
8 ounces Gruyère cheese (or other semi-hard cheese, like Asiago, Jarlsberg or sharp white cheddar), grated
*Mushrooms can be replaced with 10 ounces broccoli or cauliflower florets, blanched and blotted dry, or 1 pound frozen cut spinach, thawed and squeezed dry.
Shape the crust. Preheat your oven to 425°F. Grease a 9-inch deep dish pie plate and set aside. Remove the pie dough from the refrigerator. Place it on a lightly floured piece of unbleached parchment paper, dust lightly with flour, and roll into a 12-inch round, about 3/8-inch thick. Roll the pie crust loosely on the rolling pin and then unroll it over the prepared pie plate. Press the pie crust gently into the bottom and up the sides of the pie plate and, with kitchen shears, trim the crust so that only 1/4-inch of excess is overhanging the plate. Tuck the 1/4-inch of excess under itself, and crimp the edge gently all the way around the crust. Pierce the bottom of the pie crust with the tines of a fork and place the pie plate in the freezer to chill for 10 minutes.
Par-bake the pie shell. Remove the pie plate from the freezer and place a large piece of unbleached parchment paper in the center of the crust. Place pie weights or dried beans in a single layer in the center of the pie crust, on top of the paper. Place in the center of the preheated oven and bake for 15 minutes or until just beginning to brown. Remove from the oven and remove the paper and pie weights. Reduce the oven temperature to 350°F.
Cook the vegetables/prepare the filling. In a medium sauté pan over medium heat, place the butter and oil. Once the butter has melted, add the onion and cook for a minute, stirring frequently. Add the mushrooms, salt and pepper, and cook, stirring frequently, until the mushrooms are browned and the pan has very little if any liquid in it (about 10 minutes). Drain any remaining liquid from the pan and set it aside. In a medium size bowl, beat the eggs with the half and half or milk until well-combined. Line the bottom of the parbaked pie crust with half of the grated cheese, then the cooked mushrooms and onions, followed by the remaining cheese. Pour the milk and egg mixture on top.
Bake. Place the dish in the center of the preheated oven and bake for about 35 minutes, or until the quiche is mostly set and lightly golden brown on top, with just a tiny bit of jiggle in the center when moved from side to side. The quiche will finish cooking when it’s removed from the oven. Remove it from the oven and place the pan on top of a wire rack to cool for at least 20 minutes or until set. Slice into wedges and serve warm. Any leftovers can be covered and placed in the refrigerator for at least 3 days. Warm in a 200°F oven before serving.