makes 1 loaf; timing wise, begin autolyse before noon and you will have the bread shaped and in the refrigerator by dinner time, bake first thing next morning (remember to preheat the pot in the oven.)
Using a mixer with dough hook, combine the flour with 420 grams water. Cover bowl and allow to rest 1 hour.
Add starter to dough and mix with hook until combined. This begins the "bulk" fermentation.
Mix in the salt and dribble in the remaining water while using the dough hook. Raise the speed to get the dough to cling around the hook, but it should still stick to edge of bowl.
Let the dough hook do it's work for 5 or 10 minutes on a low speed after the dough comes together. (In my Sourdough highlight on IG, you can watch a video of the dough at this stage.) Cover bowl and allow to rest for 90 minutes.
After the 90 minute rest, I target 4 stretch and folds, one every hour. I do this using a bench scraper, working in the bowl pulling from bottom to top around the clock (noon, 3, 6, 9) just once or maybe twice.
After bulk fermentation, use a bench scraper to pull and tuck dough around and under the dough ball, shaping it with your hand and the bench scraper in the other hand until you form a taught, jiggly ball. If it's not somewhat jiggly, ferment a little longer before shaping. Let rest while you prepare the banneton/proofing basket.
Line the banneton with a linen liner then dust with flour.
Basic shaping technique involves turning the dough ball upside down then gently folding it down in thirds like a letter, then spin 180° and repeat—not being afraid of de-gasing the dough but not being too aggressive either. You want the shaped loaf to have a taught "shell" something for the air inside the dough to push against and puff and stretch in the oven.
Pinch ends closed and turn upside down into the lined banneton. Cover with plastic "shower cap" bowl cover or a slightly moist lint-free kitchen towel and set into the refrigerator for a cold proof overnight, maximum of 10-12 hours or the dough will become overly sour.
Alternatively, you can bake the dough same-day. To do this, proof at room temperature for 2–4 hours and then proceed with baking.
Place covered Dutch-oven into oven (rack lowered just under middle so that covered baker fits) and preheat to 475°F.
Tear a piece of parchment paper large enough to cradle your bread dough into the baking pot, about 15-inches. Get your spray bottle with fresh water ready, flour for dusting, and your lame.
When oven has preheated, carefully remove the covered pot from the oven.
Remove dough from refrigerator and invert onto the parchment paper. Spritz with water, and score lengthwise with the lame. Immediately transfer to the pot by using the parchment as a cradle and lowering it into the hot pot. Return the lid and place back in the oven. Bake for 28–30 minutes at 475°F (lower to 450°F if you have convection); remove lid and bake for 15 minutes longer. Immediately transfer dough to a rack to cool so that the bottom doesn't get too dark.