Feed your starter. Make sure your sourdough starter is active, bubbly, and at least doubled in size before using. You’ll need 75 g.
In a large bowl, mix your active sourdough starter and warm water. After thoroughly combined, add all-purpose flour and sea salt. Mix until a shaggy dough forms. Cover and rest for 1 hour.
Perform 4 rounds of stretch and folds, spaced 30 minutes apart. Keep dough covered between folds. After the last fold, let the dough rest at room temperature until puffed and bubbly (4–8+ hours depending on dough temperature). See Step 4 above for more details.
In a small bowl, mix brown sugar, cinnamon, and nutmeg. Chop your apples into ½-inch chunks, then toss them in the cinnamon mixture until well coated. Heat about 1 tsp of butter in a skillet over medium heat. Cook the apple mixture for 5–8 minutes, until the apples soften slightly and the sugars thicken into a syrupy coating. Let cool before using.
Line your banneton basket with a tea towel and flour generously with rice flour or all-purpose flour. Gently stretch the dough into a rectangle about 16×12 inches. Evenly spread about ¾ of the apple filling over the surface, leaving a border around the edges. Starting from one short side, gently fold the dough toward the center. Repeat on the other side so it’s folded like a letter. Carefully roll the dough into a log, then shape it into a boule or leave it as a batard. Place seam-side up in your prepared banneton.
Cover and place the dough in the fridge overnight (or for up to 2 days) for the final rise.
Place a Dutch oven with the lid on into your oven and preheat to 450°F (232°C). To help prevent the bottom of your loaf from burning, you can place a cookie sheet on the rack below. When ready to bake, cut a piece of parchment paper and gently flip your dough onto it. Score the top with a sharp blade or lame.
Carefully transfer your dough (with the parchment paper) into the hot Dutch oven. Cover and bake for 30 minutes. Then remove the lid and bake for another 10–15 minutes, or until the crust is golden brown and crisp. The internal temperature should reach 205–210°F (96–99°C).
Let your bread cool on a wire rack for at least 1 hour before slicing. Enjoy!