Place the prepared peaches in a large bowl along with the brown sugar, 3 tablespoons flour, salt, vanilla extract, and melted butter. Toss gently to combine and set aside.

Prepare the pie dough by rolling a 9" crust into a 10" circle on a lightly floured surface.

Transfer the rolled dough to a parchment-lined baking sheet by loosely rolling the dough onto a rolling pin.

Brush off any excess flour from the pie dough, if needed.

Use a pastry brush to add a layer of orange marmalade onto the entire surface of the pie dough.

Spoon the peach filling high into the center of the dough, making sure to leave an approximate 1½"-2" border around the outside of the dough.

In a small bowl, blend an egg along with a little water to make an egg wash and use a pastry brush to liberally eggwash the outside of the pie crust.

Sprinkle the entire peach crostata (especially the crust) with decorative sanding sugar (optional).

Bake in a preheated oven for 50 minutes, or until the pie crust is golden and the thickened peach juices are oozing.

Cool the crostata on the baking sheet set on a wire rack for 15 minutes and then carefully use a cake mover to move the crostata to a serving platter to cool completely before serving.