For the pastry: Sift together flour, salt, and sugar in a medium bowl. Work butter into flour with a pastry cutter or 2 knives until mixture resembles coarse meal. Sprinkle in 4-5 tbsp. ice water, working dough until it just holds together. Form into a ball, wrap in plastic, and refrigerate for at least 30 minutes. Roll out dough on a lightly floured surface into a 12" round. Preheat oven to 350°. Fit into a 9" tart pan, wrap with plastic, and refrigerate for 30 minutes.
For the filling: Cut pumpkin into large chunks and remove and discard strings and seeds. Steam pumpkin on a steaming rack in a large pot over boiling water until pulp is soft, about 30 minutes. Cool slightly and scrape pulp from skin. Set pulp aside. Meanwhile put prunes in a small saucepan. Add calvados, vanilla bean, 1⁄2 cup of the sugar, and 1 cup water and simmer over medium heat until prunes are soft and liquid has reduced by half, about 15 minutes. Set aside.
Put pumpkin, remaining 1⁄2 cup sugar, cream, eggs, cinnamon, and lemon juice in the bowl of a food processor and puree until smooth, 1-2 minutes. Line the bottom of the tart shell with a layer of prunes, pour in pumpkin puree, and bake until top is golden, about 45 minutes. Remove from oven and allow to cool. Serve with remaining prunes and sauce, and toasted almonds or pumpkin ice cream if you like.