Pulse together the flour, sugar, salt and cold butter in a food processor until the butter is reduced to the size of small peas and the mixture resembles coarse sand.
Slowly add the ice-cold water, a tablespoon or two at a time, and pulse until the mixture forms into a ball of dough.
Turn the dough onto a clean work surface and form into a disc shape with your hands. No kneading. Sprinkle both sides of the disc with flour and cover with plastic wrap. Refrigerate for about 1 hour to allow the dough to fully chill.
For the Port Syrup:
In a medium saucepan, over medium heat, bring the port, brown sugar and vanilla to a rolling boil. Reduce the flame so as not to scorch the mixture and allow to gently boil for approximately 15-20 minutes, or as long as it takes to reduce to about 2/3 cup.
To Assemble:
Preheat the oven to 425 F. (218 C.)
When the port syrup is ready, remove the stones (or pits) from the peaches and plums and slice no more than 1/2 in (1/27-cm). thick, too thin and they won't hold up when baking.
Gently coat the fruit slices in flour and mix in half of the port syrup. Gently toss to combine. Some of the fruit will lose its shape, that's ok, proceed gently to retain as much shape as possible.
Roll out the chilled dough to form a circle about 12-in (30-cm) in diameter and place on a baking sheet lined with parchment.
Arrange the fruit in a single layer, overlapping the pieces, in the center of the dough forming a circle.
Pour the remaining half of the port syrup evenly over the fruit.
Fold the edges of the crust up and over in an overlapping pleated pattern (don’t fuss with it, it’s a rustic dish, it is not supposed to be perfect); brush the edges of the crust with a beaten egg and sprinkle with coarse sugar, if desired.
Bake at 425 F. for 15-20 minutes or until the syrup bubbles and thickens and the crust takes on a golden color. Allow to cool 15-20 minutes. Slice and serve.
Notes
Very ripe fruit is tender, handle carefully. The skins of ripened peaches and plums should peel off quite easily. The amount of peaches and plums needed will depend on their size. Feel free to add a cinnamon stick to the port as it is reducing, optional.If opting to use a frozen crust, allow to come to room temperature before rolling it out.