In a large bowl, whisk together the sourdough starter, pumpkin purée, warm water, brown sugar, and melted butter until smooth.

Add the salt and bread flour, then mix until a shaggy dough forms. Knead by hand or with a stand mixer for 5 to 8 minutes until the dough is smooth and elastic. Cover and let rest for 30 minutes.

Perform 3 sets of stretch-and-folds over the next 1 1/2 hours, resting 30 minutes between each set. Keep the dough covered between folds.

Let the dough rise at room temperature until it has increased by about 50 percent and looks airy, 4 to 6 hours total depending on kitchen temperature.

Mix the swirl filling by combining the brown sugar and cinnamon in a small bowl.

Turn the dough out onto a lightly floured surface and gently pat it into a rectangle. Brush the surface with the melted butter, then evenly sprinkle on the cinnamon-sugar filling.

Roll the dough up tightly into a log, pinching the seam closed. Place it seam-side down in a lightly floured proofing basket or bowl.

Cover and refrigerate overnight, 8 to 12 hours, to develop flavor and firm the dough for scoring.

When ready to bake, preheat the oven to 475°F (245°C) with a Dutch oven inside for at least 30 minutes.

Remove the dough from the refrigerator, turn it onto parchment paper, and score the top with a sharp blade.

Carefully transfer the dough into the hot Dutch oven, cover with the lid, and bake for 20 minutes. Remove the lid and bake for another 20 to 25 minutes until deeply browned.

Cool the bread completely on a wire rack for at least 1 hour before slicing to keep the swirl neat and the crumb from becoming gummy.