In a large mixing bowl, combine flour and salt, then cut in cold butter until mixture resembles coarse crumbs, about 5 minutes. Gradually add ice water, mixing until a dough forms. Wrap in plastic wrap and refrigerate for at least 30 minutes.

Preheat your oven to 375°F (190°C).

In a skillet over medium heat, sauté chopped onions in olive oil until translucent, about 5 minutes. Add mushrooms, sage, garlic powder, thyme, nutmeg, salt, and pepper. Cook until golden brown, around 8-10 minutes. Set aside to cool.

In a large bowl, whisk together pumpkin puree, cream, and eggs until smooth. Gently fold in the cooled mushroom mixture.

Roll out the chilled dough on a floured surface to fit a 9-inch tart pan. Carefully transfer it to the pan, pour in the filling, and sprinkle with cheese if desired.

Bake for 35-40 minutes until the filling is set and the top is golden brown. Check for doneness with a gentle shake; it should jiggle slightly in the center.

Let the tart cool for at least 10 minutes before slicing. Serve warm or at room temperature.