To make the crust, add the flour, 2 tablespoons of sugar and salt to the work bowl of a food processor and pulse a few times to combine. Add the cold butter to the flour, scattering the cubes around, and pulse for 2 seconds at a time until the mixture forms small pebbles and there are no large clumps of butter remaining.
Add the ice water and pulse a few more time until the mixture is still loose, but the individual pebbles are just starting to stick together (like damp sand). Turn the dough out onto a sheet of plastic wrap and use the wrap to press the dough together into a round puck about 1-inch (25mm) thick. Place in the freezer for at least 10 minutes, or you can keep it frozen for a few weeks until you need it.
When you're ready to make the pie, put the oven rack in the middle position and preheat the oven to 400 degrees F (205 C).
Place a piece of heavy duty aluminum foil on your work surface and then cover with a piece of parchment paper. If your dough has been in the freezer for a more than 10 minutes, take it out of the freezer a little ahead of time. You want it as cold as possible, but not so cold that you can't roll it out.
Remove the pit from the peaches and slice into wedges. Add to a bowl along with the chopped rhubarb, remaining sugar and potato starch. Split the vanilla bean lengthwise and scrape out the seeds, adding them in with the peaches. Toss to distribute everything evenly.
Roll out the dough into a circle about 1/6-inch (4mm) thick and then dump the fruit in the middle of the dough into a tall mound. Use the parchment paper to help you fold up the edges of the crust back over the fruit, forming some pleats along the way.
Use the aluminum foil to form a shallow dish around the pie. Be careful not to tear a hole in the aluminum and leave enough room around the edges of the pie so the crust can breathe.
Carefully transfer the pie to the oven, placing the foil directly on the oven rack. Bake for 25-30 minutes, or until the crust is golden brown.
Serve hot out of the oven, or at room temperature with whipped cream or ice cream.