In a large mixing bowl, combine the active sourdough starter and warm water. Stir until the starter is mostly dissolved. Add the bread flour, granulated sugar, and salt. Mix with your hands or a Danish dough whisk until no dry flour remains and a shaggy dough forms. Cover the bowl with a damp cloth or plastic wrap and let rest for 30 minutes (autolyse).

Perform 3-4 sets of stretch and folds over 2-3 hours, resting 30-45 minutes between each set. To do a stretch and fold, wet your hands, grab a portion of the dough, stretch it upwards, and fold it over itself. Rotate the bowl and repeat 3-4 times per set. This builds gluten strength. After the last fold, the dough should be smoother and more elastic.

Cover the bowl and allow the dough to bulk ferment at room temperature for 4-6 hours (or until it has increased in volume by about 30-50% and feels airy). The exact time will depend on your starter's activity and ambient temperature.

While the dough is bulk fermenting (or anytime before shaping), prepare the apple filling. In a small saucepan, combine the diced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Cook over medium heat for 5-7 minutes, stirring occasionally, until the apples soften slightly and release their juices. The apples should still have some texture, not be mushy.

In a small bowl, whisk the cornstarch with 1 tablespoon of cold water to create a slurry. Stir the cornstarch slurry into the apple mixture. Continue to cook, stirring, for 1-2 minutes until the mixture thickens. Remove from heat and let cool completely. This can be made a day ahead and refrigerated.

In a small bowl, combine the rolled oats, all-purpose flour, brown sugar, and cinnamon. Add the cold, cubed butter. Use a pastry blender, two knives, or your fingertips to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Refrigerate until ready to use.

Lightly flour your work surface. Gently turn the dough out onto the surface. Stretch the dough into a rough rectangle. Spread the cooled apple filling evenly over the middle two-thirds of the dough, leaving about a 1-inch border around the edges.

Fold one-third of the dough (the long edge) over the filling. Then fold the other one-third over, creating a layered rectangle. Gently seal the ends and edges to enclose the filling. Carefully shape the dough into a taut boule or batard, being mindful not to burst the filling.

Lightly flour a banneton. Transfer the shaped dough, seam-side up, into the banneton. Cover with a plastic bag or plastic wrap and refrigerate for 10-16 hours (cold proof).

Preheat your oven to 475°F (245°C) with the Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven from the oven.

Gently turn the cold dough out of the banneton onto a piece of parchment paper. Score the top of the loaf with a sharp knife or bread lame. A simple cross or a single ear score works well.

Carefully lift the parchment paper with the dough and lower it into the preheated Dutch oven. Cover with the lid and bake for 20 minutes.

After 20 minutes, remove the lid. Reduce the oven temperature to 425°F (220°C). Sprinkle the prepared crisp topping evenly over the surface of the loaf. Continue to bake uncovered for another 20-25 minutes, or until the crust is deep golden brown and the crisp topping is golden and crunchy. The internal temperature should reach 200-210°F (93-99°C).

Carefully remove the loaf from the Dutch oven and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing. This allows the crumb to set and the flavors to meld.