In a mixing bowl, combine all-purpose flour and salt, then cut in cold unsalted butter until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough just comes together. Form the dough into a disk, wrap it in plastic, and refrigerate for at least 30 minutes.

While the dough chills, preheat your oven to 375°F (190°C).

Heat olive oil in a skillet over medium heat. Sauté onions for about 5 minutes until translucent.

Add mushrooms and cook for another 5-7 minutes until browned, then stir in sage, nutmeg, salt, and black pepper.

In a separate bowl, whisk together pumpkin puree, heavy cream, and eggs until smooth.

Fold the onion and mushroom mixture into the pumpkin mixture.

Roll out the chilled dough on a floured surface and fit it into a tart pan.

Pour the filling into the crust and optionally sprinkle with Parmesan cheese.

Bake for 35-40 minutes until the filling is set and the crust is golden brown.

Cool for 10 minutes on a wire rack before serving warm.