In a large bowl, combine the sourdough starter, all-purpose flour, whole wheat flour, and water. Stir everything together until it forms a rough dough. Let the dough rest for about 30 minutes (this step is called autolyse and helps with gluten development).

After the rest, sprinkle the salt over the dough and knead it by hand or using a stand mixer with a dough hook. Knead for about 10 minutes until the dough is smooth and elastic. You can do the "windowpane test" to check for proper gluten development: stretch a small piece of dough between your fingers. If it forms a thin membrane without tearing, it’s ready.

Cover the dough with a damp cloth or plastic wrap and let it rise at room temperature for about 4-6 hours. Every hour, give the dough a stretch and fold—gently pulling the edges of the dough and folding them into the center. This helps build structure in the dough and strengthens the gluten.

Once the dough has doubled in size, turn it out onto a lightly floured surface. Gently shape it into a round or oval loaf by folding the dough in on itself, then pinching the seams together. Flip it over and place it seam-side down into a well-floured proofing basket or bowl.

Cover the dough again and let it proof for 1-2 hours at room temperature, or refrigerate it overnight for a slower, more flavorful rise. If you’re refrigerating it, let it warm up for 30 minutes before baking it the next day.

Preheat your oven to 475°F (245°C) about 30 minutes before you're ready to bake. If you have a Dutch oven, place it inside the oven to heat up as well. This helps create a steam environment that’s perfect for the loaf’s rise and crust.

Once the dough is ready, carefully transfer it to the preheated Dutch oven or a baking sheet lined with parchment paper. Score the top of the loaf with a sharp knife or razor blade to allow it to expand during baking. Cover it with the lid (if using a Dutch oven) and bake for 20 minutes.

After 20 minutes, remove the lid and bake for an additional 20-30 minutes until the bread is golden brown and sounds hollow when tapped on the bottom. If you like a darker crust, you can bake it a bit longer.

Once baked, remove the sourdough loaf from the oven and let it cool on a wire rack for at least 1 hour before slicing. This allows the crumb to set and gives you the best texture.