Place a rimmed 11-inch by 17-inch baking sheet on the center rack of your oven. Preheat the oven to 400ºF (200ºC).
Meanwhile, place a large sheet of parchment, about the size of your baking pan, on a counter. Sprinkle a bit of flour on the paper. Place the crust on top, and roll out into a 12-inch circle (it doesn’t have to be perfect).
Arrange the peaches and blueberries in the center of the rolled out dough, leaving a 2-inch border all around.
Sprinkle the granulated sugar over the fruit.
Fold the side over, covering the fruit halfway (the center should be open, almost like a fruit volcano).
Brush the dough with the egg wash. Sprinkle the coarse sugar on top, if desired. Slide the parchment paper, with the tart on top, onto the preheated baking sheet (this will be easier to do if you remove the pan from the oven momentarily).
Bake 40 to 45 minutes, until the fruit bubbles in the center, and the crust is a deep golden color. Transfer to a wire rack, and let cool at least 20 minutes before serving.