The leaven should smell like vinegar and the bubbles on the surface should have broken, add to the water. Stir until broken up.
Add the flours and mix until a dough forms and there is no dry patches remaining. Cover and let sit for at least 30 minutes.
Sprinkle the salt over the surface of the dough. Then pinch the dough, incorporating the salt into the mixture.
With wet hands, gently release and lift the edges of the dough from the bowl and pull them into the middle. Cover and let sit for 30 minutes.
Gently grabbing the dough from the middle section, lift until it detached from the bowl. Perform the first coil fold, ensuring that the ends of the dough are now tucked under the middle.
Repeat every 30 minutes for the next 4-5 hours or until the dough is slightly puffed.
At the end of the bulk fermentation, gently pull the dough out onto a lightly floured work top. Starting from the top, stitch the dough together, pulling it into the middle from both sides of the dough alternating.
From one end, roll the dough up to create tension. Flip over, placing it on a generously floured worktop. Let rest for 15 minutes.
Flip the dough back over, seam side facing up. Stitch up the dough once more, then roll from one end to the other to create tension.
Transfer to a well floured proofing basket, keeping the seam side up.
Cover the basket with a kitchen towel, then place in a plastic bag. Use a chip clip to close the bag and place in the fridge overnight.
Place the dutch oven in the oven and preheat to 250°C (480°F) and let heat for an hour.
Take the dough out of the fridge and uncover. Cover the top of the basket with a sheet of parchment paper and gently flip the dough out onto the parchment paper, seam side down.
Brush off the excess flour from the dough. With a sharp knife or scoring razor, score the top of the loaf about 3mm (1/8 inch) deep.
Transfer parchment paper and dough to the dutch oven. Cover and let bake for 25 minutes.
After 25 minutes, remove the lid and bake for an additional 25 minutes.
Remove the dutch oven and transfer the bread to a cooling rack. Let the loaf cool for a couple hours before cutting into it. This allows the interior to continue cooking and develop nice air pockets.