In a large bowl, combine the flour and salt. Add the cold, diced butter and work it in with a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Gradually add the ice water, one tablespoon at a time, mixing until the dough comes together.

Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Preheat the oven to 375°F (190°C). Roll out the chilled dough on a floured surface, line a quiche pan or pie dish with it, and trim the edges.

Blind bake the crust: line it with parchment paper, fill with pie weights or dried beans, and bake for 15 minutes. Remove the parchment and weights and bake for 5 more minutes, until lightly golden.

Heat the olive oil in a skillet over medium heat.

Add the sliced leeks and mushrooms and sauté until softened and any liquid from the mushrooms has evaporated.

Season with salt and pepper, then let the mixture cool slightly.

In a mixing bowl, whisk together the eggs, milk, and nutmeg (if using).

Stir in the sautéed leeks and mushrooms.

Add the shredded cheese and mix until well combined.

Pour the egg and vegetable mixture into the pre-baked crust.

Bake for 30 to 35 minutes, until the quiche is set and the crust is golden brown.

Let the quiche cool for a few minutes before slicing.

Serve warm or at room temperature.

Garnish with fresh herbs like parsley or chives for a burst of color and added flavor.