Step 1: In a large bowl, combine 100g active sourdough starter, 300g warm water, and 200g pumpkin puree. Mix until well combined and the starter is mostly dissolved.
Step 2: Add 500g strong bread flour, 10g fine sea salt, 2 tsp ground cinnamon, 1 tsp ground nutmeg, 1/2 tsp ground ginger, and 1/4 tsp ground cloves. Mix with your hands until no dry spots remain and a shaggy dough forms. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (autolyse).
Step 3: After autolyse, perform your first set of stretch and folds. With wet hands, gently grab a portion of the dough, stretch it upwards, and fold it over itself. Rotate the bowl a quarter turn and repeat this motion until you've completed 4-6 stretches and folds. Cover and rest for 30 minutes.
Step 4: Repeat the stretch and fold process three more times, with 30-minute rests in between each set. After the final set, the dough should be smoother, more elastic, and have developed some strength.
Step 5: Cover the bowl and let the dough bulk ferment at room temperature (around 70-75°F / 21-24°C) for 4-6 hours, or until it has increased in volume by about 30-50% and shows signs of activity (some bubbles on the surface, jiggles when shaken). The exact time will depend on your kitchen temperature and starter strength.
Step 6: Gently turn the dough out onto a lightly floured surface. Divide the dough in half (if making two smaller loaves) or keep as one. Lightly shape each piece into a rough circle or rectangle by gently pulling the edges towards the center. Cover and let rest for 20 minutes.
Step 7: Perform the final shaping. For a round loaf, gently flatten the dough into a circle. Fold the top third down to the center, then the bottom third up to overlap. Fold the sides in, then roll the dough towards you, creating tension on the surface. Pinch the seam shut. Place the shaped dough, seam-side up, into a floured banneton basket or a bowl lined with a floured tea towel.
Step 8: Cover the banneton with plastic wrap or place it in a large plastic bag to prevent drying. Cold proof the dough in the refrigerator for 12-18 hours, or up to 24 hours.
Step 9: Preheat your oven to 450°F (232°C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven. Gently invert your cold dough into the hot Dutch oven. Score the top of the dough with a sharp razor blade or lame.
Step 10: Cover the Dutch oven and bake for 30 minutes. Remove the lid and continue baking for another 20-25 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 11: Carefully remove the loaf from the Dutch oven and transfer it to a wire rack to cool completely before slicing (at least 2 hours). Slicing warm sourdough can result in a gummy interior. Enjoy your beautiful homemade pumpkin sourdough!