In a large mixing bowl, combine the active sourdough starter, pumpkin purée, and room temperature water. Stir until smooth.
Add flour, brown sugar, salt, and pumpkin pie spice. Mix until fully combined. Cover and let the dough rest for 1 hour.
Perform 3 sets of stretch and folds, 30 minutes apart, to strengthen the dough.
Cover and allow the dough to rest at room temperature for 3 hours or until it has almost doubled in size.
Turn the dough onto a floured surface, stretch it gently without tearing, then fold and roll it into a tight ball.
Place the dough seam-side up in a floured banneton basket. Cover and refrigerate for at least 12 hours for a cold fermentation.
The next morning, place a Dutch oven in the oven and preheat to 450°F. Cut 3–4 long pieces of kitchen string and tie them together at the center so the knot sits in the middle. Lightly coat the twine with avocado oil or olive oil, removing any excess. Lay the oiled twine across the top of the dough in the banneton with the knot centered.
Place a piece of parchment paper or a silicone sling over the top of the dough and gently flip it out so the strings are now underneath. Lightly dust the top of the dough with flour, then pull the twine up and over the loaf, tying each piece loosely on top to form equal sections for the pumpkin shape. Avoid tying too tightly — the bread will expand in the oven on its own to create the pumpkin design.
Score each section of the loaf with a razor blade and carefully place the dough (with parchment) into the hot Dutch oven. Bake covered for 30 minutes, then uncovered for 15 minutes, or until the crust is golden brown.
Remove the loaf and let it cool completely on a wire rack. Cut and remove the strings, then insert a cinnamon stick into the center of the dough for the stem.
