In a large mixing bowl, combine all-purpose flour and salt, stirring to blend. Cut in cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, mixing until the dough just comes together. Shape the dough into a disk, wrap it in plastic wrap, and chill in the refrigerator for 20 to 45 minutes to firm up.

While the dough chills, heat a skillet over medium heat and add sliced mushrooms. Sauté the mushrooms until golden brown and any excess moisture evaporates, about 5-7 minutes. Season lightly with salt. In a large bowl, mix pumpkin purée, fresh sage, salt, heavy cream, and nutmeg until smooth and well combined, setting aside for assembly.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough to fit your tart pan, ensuring it’s about 1/8 inch thick. Carefully fit the rolled dough into the tart pan, pressing it into the edges. Trim any excess, then prick the bottom with a fork. Blind bake the crust for 15 minutes, or until lightly golden.

Remove the crust from the oven and pour the prepared pumpkin mixture evenly into the tart shell. Bake in the preheated oven for about 40-45 minutes, or until the filling is set and the edges are golden brown. Keep an eye on it; the center should jiggle slightly but shouldn’t look wet.

Once baked, allow the tart to cool at room temperature for about 30 minutes. This cooling time helps the flavors meld and makes slicing easier. After cooling, slice into wedges and serve warm or at room temperature.