In the large mixing bowl, combine the sourdough starter and lukewarm water. Stir to dissolve the starter. Add both flours and mix until there are no dry spots, forming a shaggy dough. Cover the bowl with a damp towel and let rest for 30 minutes (autolyse).

Sprinkle the salt evenly over the dough. Gently fold and knead the dough in the bowl for 2-3 minutes to incorporate the salt.

Let the dough rise at room temperature for 4-5 hours. During this time, perform 3 sets of stretch-and-folds every 30 minutes: grab one edge of the dough, stretch it up, and fold it over onto itself. Turn the bowl and repeat on all sides.

Dust the countertop with flour. Turn the dough out, divide it in half with a dough scraper, and gently pre-shape each piece into a ball. Let rest uncovered for 20 minutes.

Shape each ball into a tight round loaf. Place each loaf seam-side up in a well-floured proofing basket or bowl lined with a floured towel. Cover and let rise for 2–3 hours at room temperature or refrigerate overnight for up to 14 hours.

Preheat your oven to 230°C (450°F) with a Dutch oven inside, for at least 30 minutes. When ready to bake, turn a loaf out onto parchment, and score the top with a sharp knife or lame.

Carefully transfer the loaf (with parchment) into the hot Dutch oven. Cover and bake for 20 minutes. Remove the lid and bake for another 25 minutes until deeply golden and crusty.

Let cool completely on a wire rack before slicing, which should take about 1 hour.