Combine 350 g water with sourdough starter.
Stir in all-purpose and whole wheat flour to form shaggy dough.
Cover and rest for 30 mins.
Add salt and remaining 25 g water. Mix thoroughly.
Cover and rest for 30 mins.
Perform 4-6 sets of stretch and folds every 30 mins until dough rises by 30%.
Flour half your working surface.
Transfer dough to floured side, stretch into rectangle.
Fold bottom end halfway up, then top end over bottom.
Roll dough, place seam side down on unfloured side, shape into round loaf.
Dust with flour, place seam side up in floured proofing basket.
Wrap and refrigerate for 18 hours.
Preheat oven to 500°F (260°C) with Dutch oven inside.
Invert dough onto parchment-covered plate, score as desired.
Transfer dough to Dutch oven using parchment as sling.
Bake at 450°F (230°C) for 20 mins covered, then 15-20 mins uncovered or until golden brown.
Cool on wire rack for 1 hour before slicing.