Preheat oven to 400F.

In a large bowl, toss 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 350F.

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.