- Place rack in the middle of the oven. Preheat the oven to 190°C (375°F). Bake pie shell on middle rack for 10 minutes. Remove from oven and set aside.
- Meanwhile, heat oil over medium-high heat in a skillet. Add leeks and mushrooms and cook until leeks are tender, mushrooms are golden, and no liquid remains, about 8 minutes. Season with salt and pepper.
- Preheat the oven to 220°C (425°F).
- In a bowl, whisk together eggs, milk, and thyme. Sprinkle ½ cup (125 mL) Swiss cheese over bottom of pie shell. Spread vegetable mixture evenly over cheese. Pour egg mixture overtop and sprinkle with remaining cheese.
- Bake quiche in middle of the oven for 25 minutes or until a toothpick inserted in the center comes out clean.
To avoid a soggy crust, cook vegetables until the liquid has completely evaporated.