1. Make the crust:

• Combine the dry ingredients: In a large bowl, mix the flour and salt.
• Cut in the butter: Add the cold, cubed butter into the flour mixture. Using a pastry cutter, forks, or your fingers, cut the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter remaining.
• Add the water: Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. You may not need all the water.
• Chill the dough: Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes (or up to 1 hour) to firm up.
• Preheat the oven: Preheat the oven to 375°F (190°C).

2. Prepare the filling:

• Sauté the mushrooms and aromatics: While the dough chills, heat the olive oil or butter in a large skillet over medium heat. Add the chopped onion and cook for about 5 minutes, until softened. Add the minced garlic and cook for another minute.
• Cook the mushrooms: Add the sliced mushrooms to the skillet and cook for about 10 minutes, stirring occasionally, until the mushrooms release their moisture and start to brown. Season with salt, pepper, and thyme. Remove from heat and allow the mushroom mixture to cool slightly.

3. Assemble the quiche:

• Roll out the dough: On a lightly floured surface, roll the chilled dough into a circle large enough to fit a 9-inch (23cm) pie or tart pan. Gently transfer the dough to the pan and press it into the bottom and up the sides. Trim any excess dough around the edges, and crimp the edges with your fingers or a fork.
• Blind bake the crust (optional but recommended): To ensure a crisp crust, line the dough with parchment paper and fill it with pie weights or dried beans. Bake for 10-12 minutes, then remove the weights and parchment paper, and bake for another 5 minutes to allow the crust to set. This step helps prevent the crust from becoming soggy.
• Make the custard: In a large bowl, whisk together the eggs, cream, milk, salt, pepper, nutmeg, and Parmesan cheese.
• Combine: Once the mushroom mixture has cooled slightly, stir it into the egg mixture. Add the Gruyère cheese and mix until well combined.

4. Bake the quiche:

• Pour the filling into the crust: Pour the mushroom and egg mixture into the prepared pie crust, spreading it out evenly.
• Bake: Bake the quiche in the preheated oven for 30-35 minutes, or until the custard is set and the top is golden brown. A knife or toothpick inserted into the center should come out clean when the quiche is done.
• Cool slightly: Remove the quiche from the oven and let it cool for 5 minutes before slicing. This helps the filling set and makes it easier to slice.

5. Serve:

• Slice the Mushroom Quiche into wedges and garnish with freshly chopped parsley, if desired. Serve warm or at room temperature with a simple green salad on the side.