Mix the flour, water, and mature sourdough starter in a small container until smooth. Cover and let it sit at 78-80°F (26-27°C) for about 5 hours, or until it triples in volume.
While the levain is almost ready, combine the all-purpose and whole-wheat flours in a large bowl. Add water and stir until no dry spots remain. Cover and let rest for 1 hour at room temperature.
Add 65 g (about 2 1/3 ounces) of levain to the dough and incorporate it by dimpling and folding it in with wet hands. Cover and let rest for 30 minutes.
Sprinkle salt over the dough and mix it in using the same dimpling and folding technique. Transfer the dough to a baking dish, cover, and let rest for 45 minutes.
Over the next few hours, perform three rounds of coil folding (about every 60-90 minutes). This helps build structure and strength. Allow the dough to rest and proof between folds.
After the final proof, gently shape the dough into an oval and transfer it to a prepared banneton, seam side up. Cover and proof at room temperature for 15-20 minutes, then refrigerate for 12-16 hours.
Preheat your oven to 485°F (250°C) with the combo cooker inside. Score the dough with a sharp knife, transfer it to the hot cooker, and bake covered for 20 minutes. Remove the lid, reduce the oven temperature to 450°F (230°C), and bake for another 15-20 minutes until golden brown.