Pulse the flour, sugar and salt in a food processor for 5 seconds.
Add the butter and pulse until the mixture resembles coarse sand, with some small lumps of butter (no larger than pea-sized) remaining. This will probably take 15 to 20 (1-second) pulses.
Feed one tablespoon of ice water at a time through the feed tube of the food processor, pulsing 4 to 5 times after each addition. Stop adding water when the mixture begins to form large clumps and pulls away from sides of the food processor bowl.
Dump the mixture onto a work surface and push it together into a big ball using your hands.
Evenly divide the dough in half. Quickly form the halves into two balls and flatten them into 4-inch disks.
Place each disk in a plastic baggie or wrap it in plastic wrap, and refrigerate for at least an hour and up to 4 days.
Remove the dough from the refrigerator 10 minutes ahead of time so that it just starts to soften and is easier to roll.
Preheat oven to 425 degrees F.
Peach and blueberry filling
In a small bowl, beat together the egg and a splash of water. This is for the egg wash. Set aside.
Combine the peaches and peaches in a medium bowl and sprinkle with flour and sugar. Stir gently with a spatula just until combined.
On a sheet of parchment paper, roll one of the pie disc into a 12″ circle. Place the parchment paper and dough into a rimmed baking sheets.
Arrange the peaches in circles over the pie crust working from the outside in. Sprinkle or arrange the blueberries on top of the peaches.
Leave a 1.5″ to 2″ dough border. Do not pour in the juices. Dot the top of the peaches with small chunks of 1/2 tbsp butter.
Fold the edges of the gallette up and over the peaches, pinching the overlapping edges together to form a nice seal. Brush the crust with beaten egg and sprinkle with turbinado sugar.
Repeat steps 3-6 for the other galette.
Bake each galette for 18-22 minutes until the crust is golden brown and the peach juices are syrupy. Let sit 15 minutes before serving.