Mix the dough

• Combine the water and active sourdough starter in a bowl and stir until dissolved.
• Add the bread flour and mix until no dry flour remains.
• Cover and let rest.

Add salt and develop gluten

• Sprinkle the salt over the dough and work it in by folding and squeezing.
• Perform a set of stretch and folds every 30 minutes, several times, until the dough is smooth and elastic.

Bulk fermentation

• Cover the bowl and let the dough rise at room temperature until puffy and increased by about half to two thirds.

Laminate with pesto and mozzarella

• Turn the dough out onto a lightly floured work surface and gently stretch it into a large rectangle.
• Spread the pesto over the surface and scatter the cubed mozzarella evenly.
• Fold the dough over itself to enclose the fillings.

Shape the loaf

• Shape the dough into a tight round, sealing the fillings inside.
• Place seam side down in a floured banneton or bowl.

Cold proof

• Cover and refrigerate overnight.

Bake

• Preheat the oven with the Dutch oven inside to 250 °C.
• Turn the loaf out onto parchment, score the top, and lower it into the hot Dutch oven.
• Bake covered, then uncovered until deep golden and hollow-sounding.

Cool

• Transfer the loaf to a rack and let it cool completely before slicing so the cheese sets.