For the Crust
1. Place the flour, sugar, and salt in the bowl of a food processor fitted with a steel blade. Pulse a few times to combine.
2. Add the butter. Pulse 12 to 15 times, or until the butter is the size of peas. With the motor running, add the ice water all at once through the feed tube. Keep hitting the pulse button to combine, but stop the machine just before the dough comes together.
3. Turn the dough out onto a well-floured board, roll it into a ball. Wrap the disk in plastic wrap and refrigerate for 1 hour.
4. Preheat oven to 450º. Line a rimmed baking sheet with parchment paper or silicone mat. Set aside.
5. Lay a large piece of parchment paper or plastic wrap on the counter. Place another piece of parchment on top and roll out the dough to an 11 inch circle. Gently remove the top layer of parchment paper; transfer to the prepared pan. Peel off the bottom layer of parchment paper.
For the Filling
1. Add the peaches to a bowl with the blueberries, and 1 tablespoon each both of flour and sugar. Place the mixed fruit on the dough circle, leaving a 1 1/2-inch border.
2. Combine the remaining 2 tablespoons of flour and sugar, butter, oatmeal and cinnamon in the bowl of the food processor. Pulse until the mixture is crumbly.
3. Sprinkle evenly over the fruit. Gently fold the border of the pastry over the fruit, pleating it to make an edge.
4. Bake for 25 minutes, until the crust is golden and the fruit is tender. Cool for 15 minute, then transfer it to a wire rack. Serve warm or at room temperature.