For the Pie Crust:

1. In a large mixing bowl, combine flour and salt.
2. Add cold diced butter to the mixture. Use a pastry cutter or your fingers to blend the butter into the flour until it resembles coarse crumbs.
3. Gradually add ice water (1 tablespoon at a time) until the dough starts to come together.
4. Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
5. Roll out the dough on a floured surface and line a quiche pan or pie dish. Trim the edges and refrigerate while preparing the filling.

For the Filling:

1. Heat olive oil in a skillet over medium heat.
2. Add sliced leeks and mushrooms to the skillet, sautéing until softened and any liquid from the mushrooms has evaporated. Season with salt and pepper.
3. Let the mixture cool slightly.

For the Quiche Filling:

1. In a mixing bowl, whisk together eggs, milk, and nutmeg (if using).
2. Stir in the sautéed leeks and mushrooms.
3. Add shredded cheese and mix until well combined.

Assembling and Baking:

1. Preheat the oven to 375°F (190°C).
2. Pour the egg and vegetable mixture into the prepared pie crust.
3. Bake for 30-35 minutes or until the quiche is set and the crust is golden brown.
4. Let the quiche cool for a few minutes before slicing and serving.