Measure out about 1/2 cup of water and add a few ice cubes. Place into a freezer while you get the rest of the ingredients ready.
Measure out the flour, sugar and salt and place in the bowl of a food processor.* Pulse the processor a few times to combine the ingredients.
Quarter the stick of cold butter length wise and then cut across until you have approximately 12 cubes. Add the cubed butter to the flour mixture. Pulse around 8 to 10 times or until the butter is about the size of peas.
Take the ice water out of the freezer and measure out 4 tablespoons. Pour the ice water gradually through the feed tube while pulsing the processor, just until the dough starts to form some large clumps. Stop and feel the dough. If you can squeeze a piece with your fingers and it sticks together, the dough is ready. The dough should feel cold and damp but not wet and sticky. Add another tablespoon of water if the dough appears too dry. Video - How to make pastry dough with a food processor.
Dump the dough on to a clean dry work surface. Gather up the dough and form a loose ball. Place the ball on a long piece of plastic wrap. Wrap the dough and press down to form a flat disc. Refrigerate the dough for at least an hour.
Preheat the oven to 400 F. Place a piece of parchment paper on a sheet pan.
Slice the peaches and plums about 1/2 inch thick and place in a large bowl. It is up to you if you want to peel the fruit beforehand
Mix together the sugar, cornstarch, orange zest and the cinnamon in a small bowl. Add to the fruit and gently toss to combine. Set aside.
On a lightly floured work surface, roll out the prepared pastry dough until you have a 12" circle. Transfer the dough to the prepared sheet pan.
Using a slotted spoon, spoon the fruit onto the center of the pastry dough, leaving any extra juices in the bowl. Arrange the fruit over the pastry, leaving about a 2 inch border.
Lift approximately a 2 to 3 inch section of dough from the border edge and fold inwards over the fruit. Continue folding and pleating sections of dough over the fruit as you go around to make the galette. Remember, it is supposed to look a little rustic. It does not have to be perfect.
Using a pastry brush, brush some of the egg wash over the dough and then sprinkle some turbinado sugar on top. If the dough at this point is starting to warm up, you may want to refrigerate the galette 10 to 15 minutes before baking. Pastry dough needs to stay cold.
Bake the galette for 35 to 40 minutes or until the top is a golden brown and the bottom is a light golden brown. Remove from the oven and cool on a cooling rack. Serve warm or at room temperature.