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

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

Preheat your oven to 375°F (190°C) while the dough is chilling.

In a skillet over medium heat, add olive oil and sauté the diced yellow onion for about 5 minutes until translucent.

Add the mushrooms, sage, garlic powder, thyme, nutmeg, salt, and pepper. Cook until the mushrooms are browned, about 5-10 minutes. Let cool slightly.

In a separate bowl, whisk together pumpkin puree, heavy cream, and eggs until smooth. Fold in the cooled mushroom mixture.

Remove dough from refrigerator and roll it out on a floured surface until about 1/8 inch thick. Transfer to the prepared tart pan and pour the filling evenly.

Bake in the preheated oven for 35-40 minutes, or until set and golden brown. Let cool for about 10 minutes before slicing.