Whisk bread flour, whole wheat flour, and salt together in a large mixing bowl. In another mixing bowl, whisk water and sourdough starter. Pour the water mixture into the flour, and mix with your hands or a wooden spoon until a shaggy dough forms. Cover with plastic wrap, and allow to sit for 1 hour.
Begin the stretch and fold process. Grab the edge of the dough, stretch it as far as it can go without ripping, then place it on the opposite side of the dough. Repeat this four times (on the top, bottom, and sides of the dough). Allow the dough to sit for 1 hour, then continue these stretch and folds three additional times each 1 hour apart. After the last stretch and fold, allow the dough to rest for one additional hour. The dough should double in size during this time. If not, allow the dough to continue bulk fermenting until doubled in size.
Transfer the dough to a clean, lightly floured work surface, and cut into two even pieces using a bench scraper or knife. Shape the dough into a batard (oval) shape. First, flatten the dough, then grab the sides, and pinch them into the middle (this helps to create tension in the dough). Tightly roll the dough, and place it into batard proofing baskets that have been generously sprinkled with white rice flour.
Tightly, cover the baskets with plastic wrap so that they are airtight, and place in the refrigerator to proof for 18–24 hours, or overnight.
Place a dutch oven in the center rack of the oven then heat to 450°F (230°C). Allow the oven to preheat for at least 30 minutes.
Once the oven is hot, remove one basket of bread from the refrigerator. Flip the dough onto a piece of parchment paper, and score the bread with a lame or sharp knife.
Transfer the dough to the preheated dutch oven, and bake for 30 minutes with the lid on. Remove the lid, and bake for an additional 10–15 minutes until the bread is golden brown on top. Repeat this process for the second loaf.
Place the sourdough on a wire cooling rack, and allow to fully cool before slicing with a bread knife.