In a large mixing bowl, combine the active sourdough starter and warm water. Stir until mostly dissolved. Add the bread flour and salt, then mix until a shaggy dough forms.
Rest the dough
Cover the bowl and let the dough rest for 30 minutes.
Stretch and folds
Perform 4 sets of stretch and folds, spaced 30 minutes apart. Cover the dough between each set.
Bulk ferment
After the final stretch and fold, cover the dough and let it bulk ferment at room temperature for 4 to 6 hours, or until it looks puffy and has increased in size.
Shape the dough
Turn the dough out onto a lightly floured surface. Shape it into a round loaf and let it rest for 20 minutes. Tighten the shape, then place it seam side up into a floured proofing basket or towel-lined bowl.
Cold proof
Cover and place the dough in the refrigerator overnight, or for 8 to 12 hours.
Preheat the oven
The next day, place a Dutch oven in the oven and preheat to 450°F.
Score and bake
Turn the dough onto parchment paper, score the top with a sharp blade, and carefully transfer it to the hot Dutch oven. Cover and bake for 30 minutes.
Finish baking
Remove the lid and bake for another 15 minutes, or until the crust is deep golden brown.
Cool before slicing
Transfer the loaf to a wire rack and let it cool completely before slicing.
Notes
Your sourdough starter should be active and bubbly before mixing the dough.
Bulk fermentation time will vary depending on the temperature of your kitchen.
For best results, let the bread cool fully before cutting to avoid a gummy crumb.