Preheat oven to 375º Spray a sheet pan with cooking spray.
In a medium bowl, combine the flour, sugar, baking soda and salt. Cut in butter until texture resembles coarse crumbs. Stir in the currants and orange zest. Set aside.
With a fork, beat sourdough starter and buttermilk together in a separate bowl. Slowly add the sourdough/buttermilk mixture to the flour mixture. Stir until combined. The dough will be wet.
Dump the dough out onto a well-floured surface and with floured hands, knead it a few times. Form into a round loaf. Place the loaf on the prepared sheet pan, brush with egg wash if desired and lightly cut an X into the top of the bread with a serrated knife.
Bake for 45 minutes or until a cake tester inserted comes out clean or when you tap the loaf it will have a hollow sound. Let cool on a wire rack.