Preheat the oven to 375F.
On a lightly floured surface, roll your pastry out to a rough circle about 10-12 inches in diameter. Transfer to a parchment-lined baking sheet.
In a medium bowl, stir together the sugar, cornstarch and salt. Add the berries and gently toss to combine. Pour onto the middle of the pastry and fold the edges over, aiming for an inch or so around each side to contain them. If you like, dot the berries with butter, brush the edge of the pastry with milk or cream and sprinkle with coarse sugar.
Bake for 40-45 minutes, until golden and bubbling and the juices are no longer cloudy. Cool to warm before slicing and serving with ice cream or whipped cream. Serves 6-8.