Preheat the oven to 400F.
Slice the stone fruits into a large bowl. Add the berries. In a small bowl, stir together the sugar, cornstarch and salt. Sprinkle over the fruit and gently toss to coat. Dump it into the shell, shaking any leftover sugar from the bottom of the bowl overtop.
In the same bowl, blend together the brown sugar, flour, oats and butter with a fork or your fingers, until well combined and crumbly. Scatter over the fruit. Bake for 1 hour, or until bubbly and deep golden. If the crust (or topping) is browning too quickly, cover it loosely with a piece of foil.
Cool the pie completely on a wire rack before serving with ice cream or whipped cream. Serves 8.