Start by combining your ripe, active sourdough starter with the all-purpose or bread flour and water in a bowl. Cover it loosely with a cloth and let it sit at room temperature for 3-4 hours, until it has doubled in size, is bubbly, and has a peaked top.

Grate two medium-sized apples until you reach 200 grams, then set them aside. Squeeze out some excess moisture if they are very juicy.

Combine 100 grams levain, 250 grams of warm water, grated apples, 450 grams of bread flour, and 50 grams of whole wheat flour in a large bowl. Stir to moisten all the flour, then let this mixture rest for 30 minutes.

Add the reserved 10 grams of salt and 25 grams of water into the dough mixture. Incorporate the salt by squeezing and pinching the dough together.

Over the next two hours, perform a series of stretch and folds to develop gluten strength, resting the dough for 30 minutes after each fold.

After the final stretch and fold, let the dough rest in a warm spot for an additional 2-2.5 hours.

Prepare the cinnamon filling by combining softened butter, brown sugar, ground cinnamon, cornstarch, and salt.

Prepare a bowl or banneton with a liner or sprinkle it generously with rice flour.

Gently turn the dough out onto your countertop and shape it into a 12 by 12-inch square. Spread most of the cinnamon filling across the top, folding the sides of the dough towards the center and applying the remaining filling, then roll it up.

Place the shaped dough seam-side down in the prepared banneton, cover it, and refrigerate it overnight for about 18-24 hours.

Preheat the oven to 500ºF, placing your Dutch oven inside to heat.

Once preheated, carefully flip the banneton to place the dough on a piece of parchment paper. Score the top with a single cut about an inch deep.

Bake in the hot Dutch oven, covered, at 450ºF for 25 minutes. Then remove the lid and lower oven to 400ºF for another 20 minutes until the internal temperature reaches 205ºF.

Allow the loaf to cool completely on a wire rack before slicing and enjoying.