In a large bowl, combine the sourdough starter and filtered water. Stir in the flour, sugar, and cinnamon until a shaggy dough forms. Cover and let it rest for 30 minutes.

After resting, sprinkle in the salt. Over the next 2–3 hours, perform stretch and folds every 30 minutes. During the second set of folds, add the grated apple and gently work it into the dough.

Cover the bowl and allow the dough to bulk ferment at room temperature for 6–8 hours, or until it has nearly doubled in size and looks airy.

Lightly flour your work surface and shape the dough into a round. Place it seam-side up in a floured banneton or lined bowl. Cover and refrigerate for 12–14 hours.

Preheat your oven to 450°F with a Dutch oven inside. Once hot, carefully remove the Dutch oven. Place your dough on parchment paper, score the top, and prepare to bake.

Add a few ice cubes to the Dutch oven to create steam, then carefully lower in the dough. Cover with the lid and bake for 30 minutes.

Remove the lid, lower the temperature to 400°F, and bake for another 15 minutes until golden brown.

Remove the bread from the oven and let it cool on a wire rack for at least one hour before slicing. This step helps the crumb set and enhances the flavor.