Roll out your pie crust into a 9-10 inch circle that's about 1/4 inch thick.
Place the rolled pie crust on a parchment lined cookie sheet. A silicone baking mat would work, too.
In a large mixing bowl, whisk together the sugar, flour, cornstarch, and spices.
Add the sliced peaches into this mixture, and toss to coat completely.
Arrange the peaches in the center of the pie crust. You can make a pretty pattern, or keep it rustic by just "dumping" them. Fold up the edges of the crust around the peaches.
Press the Turbinado sugar into the edges of the crust. This is optional, but will give the crust a little bit of a crispness and sparkle around the edges.
Bake the galette in a 425° oven for approximately 20 minutes, or until the crust starts to turn brown.
Cool slightly before slicing. Can be served warm, or at room temp. Excellent with a scoop of vanilla ice cream or fresh whipped cream.