Preheat your oven to 425°F (220°C). Place a cast-iron skillet or a baking sheet in the oven to preheat as well.
In a large mixing bowl, combine the all-purpose flour, whole wheat flour, baking soda, and salt. If you're using raisins or currants, add them to the dry ingredients and mix well.
In a separate bowl, combine the active sourdough starter and buttermilk.
Make a well in the center of the dry ingredients and pour in the wet mixture. Stir until a sticky dough forms.
Turn the dough out onto a floured surface and knead it briefly, just until it comes together.
Shape the dough into a round loaf and place it in the preheated skillet or on the preheated baking sheet.
Use a sharp knife to score a deep "X" on the top of the loaf. This helps the bread expand while baking.
Bake in the preheated oven for about 15 minutes, then reduce the oven temperature to 400°F (200°C) and continue baking for another 25-30 minutes or until the bread is golden brown and sounds hollow when tapped on the bottom.
Remove the bread from the oven and cool on a wire rack before slicing.