In a medium bowl, combine the flour, salt, and 2 tablespoons of the granulated sugar. Add the cold butter and shortening, and cut it into the flour with a pastry blender, a fork, or your fingers until the mixture resembles coarse crumbs with some pieces of butter no larger than pea size. Whisk the egg yolk together with the cold water and sprinkle over the flour mixture, continuing to blend until the dough just comes together to form a ball; take care not to overwork the dough. The dough might seem slightly dry, but as it rests, the flour will continue to absorb liquid -- don't add too much liquid or your crust will not be flaky. Shape the dough into a disk about 1-inch thick, wrap with plastic wrap, and refrigerate for 30 minutes.

Preheat the oven to 425°F.

Remove the dough from the refrigerator and place it on a large piece of floured parchment paper. Using a floured rolling pin, roll the dough to a circle about 14 or 15 inches in diameter and about 1/8-inch thick. (Don't worry about the edges being smooth -- the charm of this tart lies in its rustic simplicity.) Lift the pastry up by grasping the edges of the parchment and transfer it to the freezer for 15 minutes.

While the dough is chilling in the freezer, cut the plums in half and remove the stones. Cut the fruit into slices about 1/3-inch thick and transfer to a mixing bowl along with the remaining granulated sugar, cornstarch, lemon zest and lemon juice. Toss to combine well.

Remove the pastry from the freezer and place it (still on the parchment paper) on a large baking sheet.

If you see any holes or cracks in the pastry, patch them by moistening the area with a drop of water and pinching a bit of dough off of the edge of the pastry and then pressing it onto the moistened dough, covering the hole(s).

Using a rubber spatula, spread the jam evenly over the center portion of the pastry. Spoon the fruit onto the center of the dough, leaving about a 3 inch border all around. Turn any sharp edges of the fruit so they will not tear the dough when you fold the edges up. Working carefully so as not to tear the pastry, lift the edges up and over the fruit on all sides. Note:  if the pastry is too cold/firm to manipulate without it tearing, just wait patiently for a few minutes until it warms up enough to bend without it breaking.

Spoon any accumulated juices remaining in the bowl over the fruit. Using a pastry brush, brush the cream over the top edges of the pastry and sprinkle with the coarse sugar.

Transfer the tart to the oven and bake for 15 minutes.

Reduce the oven temperature to 375°F, and continue baking until the pastry is golden brown and crisp and the fruit filling is glistening and bubbly, 30 to 40 minutes longer.

Remove from the oven and cool on a wire rack for at least 1 hour before serving.

Serve warm or at room temperature, with creme fraiche, whipped cream, or ice cream.