When the starter is ready to bake with, it will be light and fluffy with air bubbles, and a small pinch of it will float in water.
Scoop out 28g of sourdough starter and mix it with 138g warm water and 138g bread flour. Let it sit for 5-8 hours in a super warm place (at least 72°) until you have approximately 270g of leaven.
Mix the Final Dough. When it becomes super bubbly, mix 270g leaven, 550g water, 900g flour and gently mix until it is homogeneous. Cover your dough and let sit for 30 minutes somewhere warm.
Prepare Salt Water. Meanwhile, in another small bowl, combine 25g water with 20g salt (do not add to dough yet). After 30 minutes, add the salt water and gently fold into the dough.
Autolyse. Cover and let it sit for 1 hour somewhere warm. Then, fold 3-4 times and let sit again for 1-1/2 hours.
Bulk Fermentation. Fold 3-4 times and let it sit again—this time for two hours (somewhere warm!).
Divide, Prehape, and Bench Rest. Divide dough in half for two loaves—make sure you weigh each half so that they’re roughly equal. Let stand on the counter for 1-1/2 hours, covered with a tea towel. While it’s resting, generously flour two lined round 10” banneton bowls.
Shape. Next, fold gently into a ball. Flip it over so the seam is on the counter, and gently push and pull the dough ball until it is tight and no longer sticky but has a compressed ball-like feel.
Cold Fermentation. Using your bench scraper, quickly but gently scoop up the ball of dough, flip it, and place it into your floured-lined banneton bowl. At this point, you can either bake or cover your bannetons with a linen bowl cover and refrigerate it overnight.
If you choose to bake your loaves now, place your bannetons in a warm place for 60 minutes while you pre-heat your oven and cloche to 500° F.
When the dough is ready to put in a preheated Dutch oven or bread baker (whatever you have with a lid), bake at 450° F for 15-18 mins with the lid on. Remove the lid and bake for an additional 18 minutes. Before you cut into it, let it cool completely on a wire rack. Store your loaves in linen bread bags or freeze the baked (but cooled bread) for use later.