Preheat the oven to 400°F.
Lightly flour your work surface then stack two of the pie crusts on top of one another.
Roll the crusts into a rectangle measuring 12x17 inches. Transfer the dough into a 10x15-inch jelly roll pan, leaving the edges untrimmed.
In a large bowl, stir together the sliced peaches, brown sugar, cinnamon and cornstarch. Using a slotted spoon, transfer the peach filling into the pie crust, spreading it evenly.
Lightly flour your work surface again then use any shape cookie cutters to cut out shapes from the two remaining pie crusts. (The scraps can be re-rolled.) Arrange the shapes atop the peach filling in a single layer.
Using your fingers, crimp the pie crust around the edges.
Whisk the egg then brush it over the top and sides of the pie. Sprinkle the top of the crust with sanding sugar (optional).
Bake the pie for 28 to 35 minutes or until the crust is golden brown and cooked through.
Remove the pie from the oven and let it cool for about 30 minutes or until the juices have set. Slice and serve.