In a large mixing bowl, combine all-purpose flour and salt, then add cold, cubed butter. Mix until it resembles coarse crumbs. Gradually add ice-cold water, stirring until the dough comes together. Form it into a disk, wrap in plastic wrap, and refrigerate for at least 20 minutes.

Heat a skillet over medium heat, drizzle in olive oil, add sliced cremini mushrooms, and sprinkle lightly with salt. Sauté for about 5-7 minutes until golden. Add a splash of white wine, cook for an additional 2 minutes, then remove from heat.

In a medium bowl, combine pumpkin purée, finely chopped sage, and heavy cream. Add ground nutmeg and black pepper. Stir until fully blended, then fold in the sautéed mushrooms.

Preheat your oven to 375°F (190°C). Roll out the chilled dough to about ¼-inch thickness and transfer it to a tart pan. Pour the filling into the crust, smoothing the top.

Blind-bake for 25-30 minutes until the crust is golden brown and the filling is set. Cool for about 10 minutes before chilling for at least 30 minutes.

Once cooled, slice the tart into portions and serve. Enjoy the golden crust and creamy filling.