Prepare Tart Crust: In a mixing bowl, combine flour and salt; cut in butter until it resembles coarse crumbs. Add ice water gradually until a dough forms, shape into a disk, wrap, and refrigerate for 30 minutes.

Preheat Oven: Preheat your oven to 375°F (190°C) while the dough chills.

Cook Filling: In a large skillet, heat olive oil over medium heat. Add onions and sauté until translucent, about 5 minutes. Stir in mushrooms, sage, garlic powder, thyme, nutmeg, salt, and pepper. Cook until mushrooms are golden brown, about 8-10 minutes.

Combine Filling Mixture: In a bowl, whisk together pumpkin puree, heavy cream, and eggs until smooth. Fold in sautéed mushroom mixture.

Assemble Tart: Roll out the dough into a circle, place in tart pan, pour in the filling, and sprinkle optional Parmesan cheese on top.

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

Serve: Let the tart cool for 10 minutes, slice into wedges, and garnish with fresh sage leaves.