In a large mixing bowl, combine all-purpose flour and salt. Cut in cold unsalted butter until the mixture resembles coarse crumbs, about 5 minutes.

Gradually add ice water, mixing just until the dough holds together. Form into a disk, wrap in plastic, and chill for 30 minutes.

Preheat your oven to 375°F (190°C).

In a large skillet, heat olive oil over medium heat. Add diced onions and cook until translucent, about 5 minutes.

Stir in sliced mushrooms, fresh sage, nutmeg, and black pepper. Sauté for 5-10 minutes until the mushrooms are browned and tender.

In a medium bowl, whisk together pumpkin puree, heavy cream, and eggs until smooth. Fold in the sautéed mixture.

Roll out chilled dough to 1/8-inch thickness and transfer to your tart pan. Pour filling into the crust and sprinkle grated Parmesan if desired.

Bake for 35-40 minutes until edges are golden and filling is just set with a slight jiggle in the center.

Let cool for about 10 minutes before slicing. Serve warm.