In a large bowl, combine the all-purpose flour and salt. Cut in the cold butter until crumbly. Add ice water slowly and mix just until combined. Chill the dough in the fridge for 20-45 minutes.

Sauté the cremini mushrooms in a pan over medium heat. Add a splash of white wine and season lightly with salt and pepper. Cook until tender. In another bowl, combine pumpkin purée, fresh sage, nutmeg, and additional seasoning; mix until smooth.

Roll out the chilled dough on a floured surface into a circular shape for the tart pan. Fit the dough into the pan and blind-bake at 350°F for about 15 minutes, until lightly golden. Fill the crust with the pumpkin mixture and top with sautéed mushrooms.

Place the tart back in the oven and bake for another 30-35 minutes, until the filling is set and the crust is golden brown. Let cool for 30 minutes after baking.

Slice and serve warm or at room temperature with your favorite garnishes.