Preheat your oven to 375°F.

In a mixing bowl, combine the flour and salt. Cut in the butter until the mixture resembles coarse crumbs.

Gradually add cold water, one tablespoon at a time, mixing until the dough comes together. Form into a ball, wrap in plastic wrap, and refrigerate for 30 minutes.

In a skillet, heat olive oil over medium heat. Sauté the onion until translucent, about 3-4 minutes.

Add the mushrooms and cook until softened, about 5 minutes. Stir in the grated zucchini and cook for an additional 2-3 minutes. Remove from heat and let cool slightly.

In a large bowl, whisk together the eggs, milk, cheese, black pepper, garlic powder, and nutmeg (if using). Stir in the cooled vegetable mixture.

Roll out the chilled dough on a floured surface to fit a 9-inch pie dish. Place the dough in the dish and trim any excess.

Pour the egg and vegetable mixture into the crust.

Bake for 35-40 minutes, or until the quiche is set and the top is lightly golden. Let cool for 10 minutes before slicing.