1. First, make your quiche dough, by combining the flour and salt. Next, add chilled, diced butter and mix until the mixture resembles coarse meal. Then, add the egg and mix for a couple of seconds. With the mixer running, add the ice water (add as much as you need for a dough to form) and mix until you get a soft dough. Wrap the dough in plastic wrap and refrigerate for about an hour.
2. To make your filling, saute your shallot in 2 tbs of olive oil. Once it has softened, add the sliced mushrooms (I used Portabella and Button mushrooms) and cook until the mushrooms have softened and browned a little bit. Take them out and set them aside (place them on some paper towels to get as much grease and moisture out of them), and add the remaining tbs of olive oil to your pan, and cook your sliced leeks until they are tender. Add them to the mushrooms.
3. Take your dough out and roll it out to fit inside a 9-inch pie dish. Grease the dish, place the dough inside and prick the bottom of pastry with a fork several times, so it doesn’t rise while the quiche bakes. Cover the prepared baking dish with the crust inside with some plastic wrap and refrigerate it again until you are ready to bake the quiche.
4. In another bowl, whisk together the milk, half-and-half, salt, pepper, Dijon mustard, eggs and egg yolks. (The original recipe called for whole milk and heavy cream, but I lightened it up with fat-free milk and half-and-half). Preheat your oven to 375deg F and take your quiche crust out of the refrigerator. Evenly spread the mushrooms and leeks on the bottom, then slowly pour the milk and egg mixture over the top. Bake for about an hour, or until the crust is golden brown and the center of the quiche no longer jiggles. (I covered my quiche with foil after about 30 minutes of baking so the crust wouldn’t burn) Let cool for 10 minutes before serving. (*Note: If the quiche releases some liquid it’s totally fine, it’s just the moisture that the mushrooms release!)