Stir together the bread flour, wheat flour, salt, and yeast.
Add the sourdough starter and water and stir together until a moist and fairy shaggy dough forms. It should be just a little too wet to knead (see post photos).
Cover with plastic wrap and set aside to rest at room temperature for 12+ hours. It should rise quite a bit and become very bubbly.
Flour a work surface, then use a rubber scraper to turn the dough out. Place a little flour onto a dough scraper or rubber spatula and use them to fold the dough over itself a few times.
Sprinkle the dough with flour, cover with a clean cotton towel, and set aside to rise for another 1-2 hours.
Place a dutch oven, including the lid, into your oven, then heat the oven to 450°F.
Using a bread scraper, fold each side of the bread in on itself.
Remove the dutch oven from the oven (use potholders, it will be very hot!), remove the lid, and sprinkle some cornmeal into the bottom. Use your dough scrapers to scoop up the dough on either side and transfer to the dutch oven. Sprinkle the top with cornmeal, cover, and transfer back to the oven.
Bake for 35 minutes. Then remove the lid and bake for an additional 15 minutes uncovered.
Remove the bread from the dutch oven and allow to cool completely before slicing.
Notes
Use a sourdough starter that was fed about 12 hours before preparing the bread dough.