In a large bowl, mix 252g water and 70g starter until combined.
Add 350g flour and mix until no dry flour remains. The dough will be shaggy.
Cover and let rest for 30-45 minutes (fermentolyse).
Add Salt & Strengthen the Dough
Sprinkle 7g salt over the dough and mix it in using dimpling motions.
Perform stretch and folds: Grab one side, stretch up, and fold over. Repeat on all four sides.
Cover and let the dough rest for 30 minutes.
Bulk Fermentation (4-10 Hours at Room Temp)
Continue stretch and folds every 30 minutes for the first 2 hours.
Let the dough rest until it grows 60-80% in volume. This takes 4-10 hours, depending on room temperature. The warmer the ambient temperature is, the more to the 60% side you can lean. The colder it is, the more to the 80% side you want to go. The biggest mistake most beginners make is not bulk proofing long enough.
Shape the Dough
Lightly flour your surface (I use a mix of rice and bread flour) and gently turn out the dough.
Pull one side of the dough over the center, then pull the opposite side over.
Starting from the top, roll the dough down towards you to create surface tension. Be gentle and don't compress the dough too much.
Place the shaped dough into a floured proofing basket, seam-side up.
Cold Proof
Cover and refrigerate for 8-12 hours (overnight).
Bake
Preheat the oven to 250°C with a Dutch oven inside.
Turn the dough out onto baking paper/ non-stick mat, score the top, and place it in the Dutch oven.
Cover and bake for 20 minutes.
Remove the lid, turn the temperature down to 220°C and bake uncovered for another 20-25 minutes, until golden brown.
Cool & Enjoy
Let your sourdough loaf cool completely (at least 1 hour) on a wire rack before slicing. If you slice before it cools your crumb will be gummy. Be patient and don't ruin all your hard work at the last stage!