In a big bowl, stir together the starter, warm water, flour, and salt until there’s no dry flour left. Cover and let it rest for 30 minutes.
Wet your hands. Grab one edge of the dough, stretch it up, and fold it over. Turn the bowl and repeat on all sides (4 total folds). Cover and let it rest 30 minutes.
Do that same stretch-and-fold step 3 more times over the next 2 hours—every 30 minutes.
After the final fold, add your cinnamon and raisins and knead the dough a few times. Gently shape the dough into a ball. Place it seam-side up in a bowl lined with a floured tea towel. Cover and stick it in the fridge for 6–24 hours.
When you’re ready to bake, preheat your oven to 500°F with a Dutch oven inside. Let it heat for 30 minutes.
Carefully turn the dough into the hot Dutch oven (use parchment if you want). Score the top with a sharp knife. Cover and bake at 450°F for 30 minutes.
Take the lid off and bake for another 15 minutes, until the crust is golden brown.
Let your cinnamon raisin sourdough cool on a rack for at least an hour before slicing. It’s worth the wait!