In a large bowl, combine the active sourdough starter with water and mix well until the starter is mostly dissolved.

Add both all-purpose flour and whole wheat flour to the bowl, then mix them with a dough whisk or your hands until no dry spots remain.

Cover the bowl and let the dough rest for 30 minutes to an hour for autolysis, allowing the flour to fully hydrate.

After the autolysis, add salt to the dough and gently knead it in for 2-3 minutes to ensure even distribution.

Perform a series of stretch and folds every 30 minutes for the first 2 hours, then reduce to every hour for another 2-3 hours.

Allow the dough to bulk ferment at room temperature (75-78°F / 24-26°C) until it has increased in volume by about 30-50% and feels airy.

Gently turn the dough out onto a lightly floured surface and pre-shape it into a round or oval, resting it for 20-30 minutes.

Perform the final shaping and transfer the dough to a floured banneton or a bowl lined with a floured towel, seam-side up.

Proof the dough in the refrigerator for 12-18 hours, or until it is well-risen and passes the poke test.

Preheat your oven to 475°F (245°C) with a Dutch oven inside for at least 30-60 minutes.

Carefully transfer the cold dough from the banneton into the hot Dutch oven, score the top with a sharp blade, and cover.

Bake covered for 20 minutes, then remove the lid and continue baking for another 20-25 minutes until the crust is golden brown and the internal temperature reaches 200-210°F (93-99°C).

Cool the bread on a wire rack for at least 1-2 hours before slicing to allow the crumb to set and prevent gumminess.