Mix the flour and 1 1/2 cups of the water in a large bowl until a shaggy dough forms. Cover the bowl with a lid or plate and let stand until the flour absorbs the water, 30 to 45 minutes.
Add the sourdough starter, salt and remaining 3 tablespoons water to the dough. Squeeze and twist with your hands until a sticky dough forms (it will be soupy at first, but continue mixing). Knead the dough in the bowl by scooping under the dough, stretching it up and twisting it while you rotate the bowl a quarter turn; slap down the dough and repeat. Continue kneading until the dough smooths out and pulls away from the bowl, 5 to 7 minutes. Scrape down the sides of the bowl and cover with a lid. Let rest in a warm place, 30 minutes.
Uncover the dough. Gently reach under the dough with a wet hand, stretch it up as far as it will go without tearing, then fold the dough over itself. Rotate the bowl a quarter turn and repeat three times; this is a set of turns. Do another set of turns, then re-cover the bowl and return to a warm place to rest, 30 minutes.
Do two sets of turns every 30 minutes until the dough has increased in size by one-third and has large bubbles in spots, 2 1/2 to 3 1/2 more hours, covering the bowl after each time. After the last set of turns, cover the bowl and let the dough rest another 30 minutes.
Turn out the dough onto a generously floured work surface. Slide a bench scraper or large spatula underneath and fold the dough in half. Cup floured hands around the dough and gently roll it in a circular motion into a taut ball. Let rest, uncovered, 20 minutes (the dough will flatten a little).
Line a medium bowl with a clean lint-free towel and dust generously with flour. Lightly dust the top of the dough with flour, then slide a bench scraper or large spatula underneath and flip it over. Working gently to not deflate the dough, pull each of the four “sides” of the dough up and into the center, folding the last side to cover the whole top of the dough. Roll the dough over so the seams are on the bottom. Slide a bench scraper or large spatula underneath and gently invert the dough into the towel-lined bowl with the seam up. Cover loosely with the overhanging towel. Let rest in the refrigerator, at least 8 hours and up to 18 hours.
About 30 minutes before baking, place a 10- to 11-inch Dutch oven with a lid in the middle of the oven and place a baking stone or large cast-iron skillet on the bottom rack to absorb the heat; preheat to 450˚ F. Carefully remove the Dutch oven from the oven and uncover. Remove the dough from the refrigerator, uncover and lay a large round of parchment on top. Invert the dough onto the parchment. Carefully lower the dough (on the parchment) into the hot Dutch oven. Using kitchen shears or a sharp knife, make a few 1/4- to 1/2-inch-deep cuts in the top so that the dough can expand.
Transfer the pot to the oven; cover with the lid. Bake until the bread has risen and the crust is pale, about 20 minutes. Uncover and bake until the crust is browned and crisp and a thermometer inserted into the center registers 210˚ F, 25 to 35 minutes. Let cool 10 minutes in the pot, then transfer to a rack to cool completely, about 2 hours.