Mix the water, flour, and sourdough starter in a bowl until flour is incorporated
Turn out and knead for a few minutes until the levain is stiff and smooth
Place in a lightly oiled bowl and cover with plastic wrap overnight
Prepare the Dough
Place the water in a large mixing bowl or the bowl of a stand mixer
Tear the levain into small pieces (it will be sticky!) and put into the water
Add flours and salt to the water and bring together until the flour is incorporated
Either use a dough scraper to knead by hand via the pull and fold method or knead in a mixer with hook attachment on low speed for two minutes
Cover with plastic wrap and allow to rest 30 minutes
Either use a dough scraper to knead by hand via the pull and fold method for 3 minutes or knead in a mixer on speed 2 for 1.5 minutes
Place the dough in a large greased bowl and cover with plastic wrap. Allow to rise for 1 hour (I left mine for 1.5 - 2 hours and it was fine...)
Fold the dough into itself to deflate slightly then cover and leave for another hour
Gently deflate the dough
Turn out onto a floured surface and cut the dough in half with a bench scraper
Shape each half into a round and pinch the seams
Flip seam side up and cover with plastic wrap
Rest the dough for 20 minutes
Shape the loaves into final form
Place in a shaping container, if desired. This can be in a floured brotform or bowl with a floured cloth seam side up, or in an oiled cloche or Dutch oven seam side down. You can also just set them on greased parchment paper and cover with greased plastic wrap.
Cover and let rise 2 hours
Preheat the oven to 450F / 230C with the baking stone (if using) and a cast iron or (sacrificial) broiler pan inside
Boil some water on the stove top while the oven heats if you're planning to bake on a stone
Slash the loaves with your preferred pattern just prior to baking then spray with a little water
Place the loaves on the preheated stone (or the Dutch oven or cloche into the oven) and put about a cup and a half of boiling water in the preheated pan then close the oven quickly
Remove the Dutch oven or cloche lid after 20 minutes
Bake for a total time of about 40 minutes until the bread is acorn brown and internal temperature reads 195F / 90C