Mix starter and water with a fork until distributed. Add flours and salt. Mix with fork until dry, then finish incorporating flours with hands until no flour streaks remain.
Let rest, covered, for 30 minutes. This is the autolyse stage.
Then perform 4 sets of stretch and folds, every 30 minutes.
When that is finished (2 hours in), the bulk fermentation will start. Let rest, covered until doubled in size and dough is a bit jiggly when you move the bowl. In 80 degree F temperatures, this took 3 hours for me.
Gently move dough onto a lightly floured surface. Let rest for 15 minutes, covered. Then perform shaping, into a round. Let rest 2 minutes, and tighten the round by cupping your hands around it and pulling towards you, creating tension.
Using a bench scraper, place dough into banneton, seam side up. (So you'll be flipping the dough from how it is sitting on the counter, as seam side is down.)
Cover with a floured towel, and then a plastic bag, and refrigerate for 8-12 hours. The longer it stays in the refrigerator, the more tang (sour) you will get from the loaf.
Preheat oven to 475 degrees F. Remove dough fro refrigerator and score according to your preference. Place into a dutch oven, on a piece of parchment paper. Place in oven when ready, and turn down heat to 450 degrees. Bake for 20 minutes with lid on, then remove lid and bake for additional 20-25 minutes, until crust is golden brown.
Remove from oven and place on cooling rack. Let rest for one hour before slicing.