Place your bowl on a kitchen scale and measure out 730 grams of water.
Zero out the scale and add 200 grams of starter.
Mix well until it looks milky.
Add all the flours. Mix until a shaggy dough forms.
Let it sit for 40-60 minutes.
Sprinkle 20g of salt over the dough and pour in the additional 50g of water. Use your hands to mix until the salt is evenly distributed throughout the dough.
Bulk Fermentation: Stretch & Folds
Cover the dough and let it ferment at room temperature for 4-6 hours. Perform a series of stretch and folds every 30 minutes for the first 2 hours, then once an hour for the remaining time. This strengthens the dough and builds structure. Be careful here as you stretch and fold as you want to encourage gluten development and not tear the dough.
Shape & Overnight Cold Ferment
Once the dough has risen and developed structure, shape it into a round/oval boule. Place it in a well-floured proofing basket. Cover and refrigerate overnight or for a few hours for enhanced flavor and texture.
Bake Your Sourdough Boule
Preheat your oven to 500°F (260°C) with a cast iron Dutch oven or combo cooker inside.
Transfer the dough onto a piece of parchment paper, score the top with a sharp blade to allow for controlled expansion, and carefully place it into the preheated Dutch oven. Cover with lid.
Place the Dutch oven back in the oven and lower the oven temperature to 450°F (232°C) It helps to place a baking sheet on the bottom rack under the Dutch oven to keep the bottom crust of the bread from burning. It really works!
Bake covered for 30 minutes.
Remove the lid and bake for another 10-15 minutes until the crust is deeply golden with slightly burnt bits on top.
Let cool completely on a wire rack before slicing.
Video
Notes
To make this with all purpose white flour or bread flour, the ratios only change a little. 700 grams of water, 200 grams starter, 100 grams of whole wheat flour, 900 grams all purpose or bread flour, 20 grams of salt and 50 more grams of water.