Mix 50g active sourdough starter with 50g bread flour and 50g room-temperature water. Stir well, cover loosely, and leave at room temperature for 4–8 hours until bubbly and doubled.
Combine 450g bread flour, 325g water, and 10g salt in a large bowl. Add the ripe starter and mix until no dry flour remains. Cover and rest for 30 minutes.
Over the next 3 hours perform 4 sets of stretch and folds every 30 minutes: grab one side of the dough, stretch it upward, and fold it over to the other side. Rotate the bowl and repeat 4 times per set.
Turn the dough onto a lightly floured surface. Shape into a tight round by folding edges to the centre, then flip seam-side down and drag toward you to build tension. Place seam-side up in a floured proofing basket.
Cover the basket with a plastic bag and refrigerate for 12–16 hours. Cold proofing develops flavour and makes scoring easier.
Preheat oven to 250°C (480°F) with a Dutch oven inside for 1 hour. Turn cold dough onto parchment, score the top with a sharp blade, then bake covered for 20 minutes. Remove lid and bake 20 more minutes until deep brown.