In a food processor, combine flour, sugar and salt and pulse a few times to mix. Add the butter and pulse until the crumbs are about the size of a pea.

With the processor running, add 3 tablespoon ice cold water and process until the dough starts to come together into a ball. Add an additional tablespoon of water if dough isn't coming together.

Dump the dough onto a floured surface and using your hands, bring together into a disc. Wrap the disc in plastic wrap and place in the fridge for 30 minutes to chill.

Preheat the oven to 375 degrees. Line a rimmed baking sheet with foil and then place a piece of parchment on top of foil.

Combine the peaches, sugar, vanilla, peach preserves and cornstarch in a medium bowl. Mix gently to combine.

Roll out the dough onto a floured surface to a circle about ¼ inch thick and 14 inches in diameter. Place the crust onto the baking sheet.

Pour the filling onto the crust, leaving about 2 inches at the edge. Fold the edge of the crust up over the peaches all the way around the galette, pleating the dough as necessary to form a circle.

Whisk the egg in a small bowl and add a teaspoon of water. Brush the edges of the galette with the egg wash and then sprinkle with coarse sugar.

Place in the preheated oven and bake until the filling is bubbly and crust is golden brown, about 35-40 minutes. The galette will leak some during cooking. Allow to cool before slicing.