Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
In a large bowl, gentle toss the blackberries, granulated sugar, cornstarch, orange zest, and spices to combine. Set aside for 15 minutes to let the juices out.
Roll pie dough and place on the prepared baking sheet. Place the berry filling (exclude the juices which have run out so your galette isn't too runny) in the center of the dough, leaving a 2-inch border. Gently fold the dough loosly onto the filling to form the galette. Brush the dough with the egg wash and sprinkle turbinado sugar.
Bake the galette until the crust is golden brown and the juice from the berries is bubbling, 25 to 30 minutes. The juice may ooze from the edges of the pie crust. Enjoy warm (I recommend garnishing with mint and serving with ice cream).