In a large mixing bowl, combine the bread flour, water, and sourdough starter. Mix until a shaggy dough forms. Let it rest for 30 minutes to autolyse.
After autolyse, add the salt and knead the dough for about 10 minutes until smooth and elastic. Place the dough in a lightly oiled bowl, cover, and let it ferment at room temperature for 4-6 hours, or until it has doubled in size.
While the dough ferments, prepare the filling. Mix the diced apples, brown sugar, cinnamon, and melted butter in a bowl. Set aside.
Once the dough has fermented, turn it out onto a lightly floured surface. Roll it into a rectangle about 12x18 inches. Spread the apple mixture evenly over the dough, leaving a 1-inch border on all sides.
Starting from the long side, roll the dough tightly into a log. Pinch the seam to seal. Place the log seam-side down into a greased 9x5 inch loaf pan. Cover and let it proof for 2-3 hours, or until it has risen about 1 inch above the rim of the pan.
Preheat your oven to 450°F (230°C). Place a baking stone or an inverted baking sheet in the oven to preheat. Once the oven is hot, score the top of the dough with a sharp knife or razor blade.
Bake the bread on the preheated stone or sheet for 15 minutes at 450°F (230°C). Then, reduce the temperature to 375°F (190°C) and bake for an additional 30 minutes, or until the internal temperature reaches 200°F (93°C).
Remove the bread from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.