To make the levain or 'sponge', combine the 30 grams sourdough starter (or 1/4 teaspoon yeast),120 grams of all-purpose flour, and 130 grams of warm water in a bowl with a lid or storage container. Mix completely and set aside (covered) for 3 hours to activate. Alternatively, you can use 1/4 teaspoon of active dry yeast in place of the sourdough starter.
Once the levain is bubbly and activated, add in the sorgum and additional 300 grams of warm water. Stir to combine. If using yeast instead of sourdough starter, add an additional 1/4 teaspoon of yeast at this time.
Add in the whole grain flour, all purpose flour, and salt. Again, stir to completely combine. Cover and let sit for 15 minutes.
Dump the dough onto a gently floured surface. Use a dough scraper to fold the dough in on itself from each side. Place the bowl upside down over the dough and let it rest for another fifteen minutes. Repeat the dough scraping and rest again. Repeat once more. The dough will now begin to hold its shape.
Transfer the dough into a floured bowl with a lid (or cover) and let it rest at room temperature for 3-5 hours or until it's grown in volume by about 50%.
Carefully scrape the dough back onto the work surface. Gently shape the dough by grabbing the edges of the dough and pressing them back into the center with your fingertips. Transfer the loaf to a heavily floured proofing basket. Cover the basket with plastic wrap and let proof at room temperature for 1 hour.
Preheat a Dutch Oven and its lid in a 475 degree oven for 1 hour.
When you’re ready to bake the bread, remove the Dutch Oven from the hot oven. Remove the lid. Carefully flip the proofing basket into the Dutch oven to transfer the bread in. Cut the top with a razor blade or sharp knife if desired. Replace the lid and bake the bread for 45 minutes.
Remove the Dutch Oven from the oven and remove the bread to a wire cooling rack. Let cool for at least an hour before slicing.