Preheat your oven to 428F (220C). Line a baking tray or cookie sheet with parchment paper and set aside.
Add the all purpose flour, whole wheat flour, salt and baking soda into a large mixing bowl and stir to combine.
In a separate bowl, combine the buttermilk and sourdough starter and stir until it's bubbly and well combined.
Pour the buttermilk and sourdough starter liquid into the dry ingredients and quickly mix to form a sticky dough.
Lightly dust your counter top with flour and tip the dough out. Knead the dough around 8 to 10 times. Less is more with this bread.
Now form the dough into a rough round and place onto the parchment lined baking tray.
Press the dough down to around 1" thick and then slash a cross on the top using a sharp knife or lame.
Bake for 20 minutes at 428F (220C) and then turn the oven down to 392F (200C) and bake for a further 20 minutes.
Allow to cool slightly before slicing. This bread is best eaten warm.