1Preheat the oven to 400F.
2Get out a deep baking dish (mine was about 7"x10") and slice your peaches into it, not bothering to peel them. Add the blackberries. Stir the sugar, cornstarch and salt in a small dish and sprinkle overtop, then toss with your hands until more or less combined.
3On a lightly floured countertop, roll the pastry out until roughly an inch bigger around than the edge of the baking dish. Place it overtop, fold the edges under and kind of crimp them, pressing up against the edge of the pan. Don't worry about being neat about it - this is what they call rustic.
4Cut a few slits in the top and if you like, brush with milk or cream and sprinkle with coarse sugar. Bake for 20-30 minutes, until golden and bubbly. Let cool for 20 minutes to allow the juices to thicken.