In a large bowl, mix the bread flour and water until no dry bits remain. Cover and let it rest for 30-60 minutes.

Add the active sourdough starter and salt to the bowl. Use wet hands to squish and fold the dough until everything is fully incorporated.

Cover the dough and let it rest in a warm place. Perform four sets of stretch and folds, spaced 30 minutes apart, over the first 2 hours of fermentation. Let the dough continue to rise until it's airy and has grown by about 50%.

Turn the dough onto a lightly floured surface and gently stretch it into a large rectangle. Evenly sprinkle the cubed mozzarella and chopped pepperoni over the surface. Fold the dough like a letter in thirds, then fold it in thirds again to create a neat package.

Gently form the dough into a round ball (boule). Let it rest for 20-30 minutes on the counter, then perform a final shaping to create surface tension. Place the loaf seam-side up into a well-floured banneton.

Cover the banneton and refrigerate for 12-24 hours to develop flavor.

Place a Dutch oven in your oven and preheat to 450°F (232°C). Turn the cold dough out onto parchment paper, score the top, and carefully lower it into the hot Dutch oven.

Bake covered for 25 minutes. Remove the lid and bake for another 15-20 minutes, until the crust is a deep, golden brown and the internal temperature reaches at least 205°F (96°C).

Let the bread cool completely on a wire rack for at least 2 hours before slicing. This is crucial for the crumb to set.