1. Combine Starter and Water: In a large mixing bowl, gently whisk together the active sourdough starter and warm water until the starter is mostly dissolved. The mixture should look milky and slightly frothy.

2. Add Dry Ingredients and Mix: Stir in the salt, bread flour, whole wheat flour, rolled oats, flax seeds, sunflower seeds, and optional honey or maple syrup. Mix with a sturdy spoon or your hands until all the flour is hydrated and a shaggy dough forms. Don’t worry about it being perfectly smooth at this stage. Cover the bowl with a damp towel or plastic wrap and let it rest for 30 minutes (autolyse).

3. Perform Stretch and Folds: After the rest, begin the stretch and fold process. With wet hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the center. Rotate the bowl a quarter turn and repeat this action three more times until you’ve completed a full round. Repeat this set of stretch and folds every 30 minutes for a total of 4-6 rounds, or until the dough feels stronger and more elastic.

4. Bulk Fermentation: After the stretch and folds, cover the bowl again and let the dough bulk ferment at room temperature for 3-5 hours, or until it has increased in volume by about 30-50% and shows signs of activity (small bubbles on the surface). The exact time will depend on your room temperature and starter strength.

5. Shape the Boule: Gently turn the dough out onto a lightly floured surface. Carefully preshape it into a round by gently pulling the edges towards the center to create tension on the surface. Let it rest for 20 minutes. Then, perform the final shaping by gently tightening the dough into a taut boule. Place the shaped dough seam-side up into a floured banneton (proofing basket) or a bowl lined with a floured tea towel.

6. Cold Proofing (Optional but Recommended): Cover the banneton with plastic wrap and refrigerate for 8-12 hours. This slow cold proof develops flavor and makes the dough easier to handle. Alternatively, you can proof at room temperature for 1-2 hours until it passes the “poke test” (a gentle poke leaves a slow-rebounding indentation).

7. Preheat Oven and Bake: Preheat your oven to 450 F (230 C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven, invert the dough into it, and score the top with a sharp razor blade or knife. Bake covered for 30 minutes, then remove the lid and bake for another 10-15 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210 F (93-99 C).

8. Cool Completely: Transfer the baked bread to a wire rack and let it cool completely for at least 1-2 hours before slicing. This is crucial for the crumb structure and flavor to set properly.