In a large bowl, mix all of the dough ingredients together (bread flour, whole wheat flour, water, sourdough starter, pumpkin puree, salt and pumpkin pie spice) until no dry flour remains. Cover.
Do 3–4 rounds of stretch and folds, about 30 minutes apart.
Cover and bulk ferment for 5–10 hours, depending on dough temperature (about 8–9 hours at 70°F). It's ready when it jiggles like Jell-O, has bubbles on the surface, and a gentle poke doesn't spring right back. If unsure, let it go a little more proofed rather than underproofed.
Stir the brown sugar and cinnamon together.
When the bulk ferment is fully done, turn the dough out and gently stretch it into a rectangle. Sprinkle the brown sugar and cinnamon evenly over the top.
Fold the dough in thirds toward the middle (a three-fold), then roll it up into a log.
Dust a banneton with flour and place the dough in it seam side up. Cover and cold proof in the fridge overnight, 10–12 hours.
Place the Dutch oven, lid on, in the oven and preheat to 450°F along with the oven, about 10–15 minutes. Don't let it sit in the hot oven long after preheating, or the bottom of the loaf can burn.
Turn the cold loaf out onto parchment paper and score the top.
Use the parchment as a sling to lower the loaf into the hot Dutch oven. Place about 3–6 ice cubes around the outer rim, between the parchment and the pot wall, for steam.
Bake at 450°F for 30 minutes with the lid on, then remove the lid and bake 15 minutes more.
Check the internal temperature with an instant-read thermometer: it's done at 200–205°F.
Cool on a rack for at least 1 hour, until completely cool, before slicing, or the inside will be gummy and not fully set.