Using your food processor, combine the flour, salt and butter and pulse for about 5 seconds. Sprinkle the ice water over the flour mixture and pulse until the pastry dough just begins to come together, for about 10 to 15 seconds; you should still be able to see small pieces of butter in it.
Transfer the pastry dough to a lightly floured work surface, gather it together to combine everything and roll out to a 12 inch circle. Transfer the dough to a tart pan (I highly recommend the ones with removable bottoms) and refrigerate while you prepare the filling (You can also roll out the dough and use it right away if needed).
Preheat your oven to 400 degrees. Heat the olive oil in a large sauté pan over medium-high heat. After a minute or two, add the leeks and cook for about 3 minutes until softened.
Add the mushrooms, garlic, rosemary and sage and season with salt and pepper. Cook for about 5 minutes, stirring occasionally.
Add the white wine (or chicken stock) and stir to pick up the lovely browned bits on the bottom of your pan. Let the wine cook for about a minute or 2. Remove the pan from the heat and allow to cool slightly. Remove your pastry crust from the refrigerator and set aside.
Using a medium-sized mixing bowl, whisk together the eggs, heavy cream, Gruyère, parsley and thyme. Fill the tart pan with the cooled mushroom mixture. You can smooth down so it evenly fills the pan. Pour in the egg mixture over the mushrooms and leeks. Spread evenly again and sprinkle on a little more chopped fresh parsley if desired.
Bake for about 25 to 30 minutes or until cooked through and the quiche is a nice golden brown. Let cool for at least 15 to 20 minutes before serving.