Preheat oven to 375 degrees. In a small mixing bowl whisk together 3/4 cup granulated sugar, cornstarch,cinnamon,nutmeg and ginger. In a large mixing bowl,toss peaches with orange juice and optional orange zest. Sprinkle sugar mixture over peach mixture and toss to evenly coat,set aside.
In a separate large mixing bowl,whisk together flour,baking powder and salt. Cut butter into flour mixture with a fork or pastry cutter until mixture resembles coarse crumbs. In a small bowl,whisk together egg,sour cream and vanilla extract. Pour egg mixture into flour mixture and stir until well combine.
Add in remaining 1 cup granulated sugar and stir until combine. Spread and press half of crumb mixture into an even layer in a greased 13x9 inch baking dish. Toss peach mixture once more and pour over bottom crumb layer and spread peaches into an even layer (spread juices evenly too).
Sprinkle remaining crumb mixture over top of peaches into an even layer. Sprinkle top evenly with optional Turbinado sugar. Bake in preheated oven 45-50 minutes until golden. Serve warm with optional vanilla ice cream or allow to cool and cut into bars. Store bars in an airtight container in the refrigerator.