Combine the active sourdough starter, bread flour, whole wheat flour, and warm water in a small jar or container. Mix until well combined. Cover loosely and let it sit at room temperature (70-75°F / 21-24°C) for 8-12 hours, or until it has doubled in size, is very bubbly, and passes the float test (a small spoonful floats in water).

In a large mixing bowl, combine the bread flour, whole wheat flour, water, and pumpkin puree. Mix with a dough scraper or your hands until no dry spots of flour remain and a shaggy dough forms. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes to 1 hour. This step allows the flour to fully hydrate.

Add the active levain, salt, pumpkin pie spice, and optional maple syrup/brown sugar to the autolysed dough. Mix thoroughly by hand, squeezing and folding the dough until all ingredients are fully incorporated. The dough will be sticky at first. Mix for about 5-7 minutes until the salt and levain are evenly distributed. If using, fold in pumpkin seeds.

Cover the bowl and let the dough rest for 30 minutes. Begin the first set of stretch and folds. With wet hands, grab a portion of the dough from one side, stretch it up, and fold it over the center. Rotate the bowl 90 degrees and repeat this action 3-4 times until you've worked your way around the dough. Repeat this process for a total of 4 sets, spaced 30 minutes apart.

After the 4 sets of stretch and folds, let the dough rest, covered, at room temperature for an additional 2-3 hours (total bulk fermentation time will be 4-5 hours, depending on your room temperature and levain strength). The dough should have increased in volume by about 20-30% and show signs of activity (small bubbles).

Lightly flour your work surface. Gently turn the dough out onto the surface. Using a dough scraper, gently shape the dough into a loose round. Allow the dough to rest, uncovered, for 20-30 minutes. This helps the gluten relax.

Lightly flour the top of the preshaped dough. Flip the dough over. Gently stretch out the dough into a rough rectangle. Fold the bottom third of the dough up to the center, then fold the top third down over the center. Turn the dough 90 degrees and repeat the folding process, creating tension on the surface. Pinch the seam closed. Carefully transfer the shaped dough, seam-side up, into a well-floured banneton basket (or a bowl lined with a floured kitchen towel).

Cover the banneton with plastic wrap or a shower cap and place it in the refrigerator for 12-18 hours (or up to 24 hours). This slow, cold proof develops flavor and makes the dough easier to handle and score.

Preheat your oven to 500°F (260°C) with the Dutch oven inside for at least 30-45 minutes. Carefully remove the hot Dutch oven from the oven. Gently invert the dough from the banneton directly into the hot Dutch oven. If desired, sprinkle with extra pumpkin seeds.

Using a lame or a sharp razor blade, score the top of the dough. You can do a simple cross, a square, or a more intricate pattern. Immediately place the lid back on the Dutch oven and bake for 20 minutes.

After 20 minutes, carefully remove the lid from the Dutch oven. Reduce the oven temperature to 450°F (232°C) and continue to bake for another 25-30 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).

Carefully remove the bread from the Dutch oven and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing. Slicing too early can result in a gummy texture.