Method:
Preheat oven to 400°F. In a large bowl, toss cut mushrooms with onion, olive oil, salt and pepper, then place on a rimmed baking sheet in a single layer. Roast for about 15 minutes, or until cooked and firm. Remove from oven and set aside.
Prick pie shell all over with a fork. Prebake shell for 10 to 15 minutes, until lightly browned. Reduce oven temperature to 350°F.
Whisk together heavy cream and eggs with thyme, salt and pepper. In a large bowl, combine roasted mushrooms, onion, gruyere and thyme. Spread this mixture in the bottom of the pie shell in an even layer.
Pour cream-egg mixture over the mushroom mixture, ensuring that all of the mushroom mixture is covered.
Bake for 45 to 55 minutes, or until a knife inserted into the center of the quiche comes out clean. Serve warm or at room temperature.