In a clean jar, mix 25g of your mature starter with 75g of unbleached bread flour and 75g of warm water at 90°F. Stir until combined, then cover loosely. Let rest overnight for 8-12 hours.

In a mixing bowl, combine 100g of the ripe starter, 300g of warm water, and 500g of unbleached bread flour. Stir until it forms a shaggy dough and cover to prevent drying.

Let the covered bowl rise in a warm area for 5-6 hours until puffy and bubbly.

Dust a clean surface with flour, turn out the sticky dough, and perform a series of folds. Shape it into a tight ball and prepare for the second rise.

Place the shaped dough seam-side up in a floured proofing basket. Cover and let rise for 2-3 hours until noticeably larger.

Preheat oven to 450°F with a Dutch oven inside. Carefully turn the risen dough onto parchment paper, score the top, and place it in the hot Dutch oven.

Cover with the lid, reduce the temperature to 425°F, and bake for 40 minutes. Remove the lid and bake for an additional 5-10 minutes until golden brown.

Transfer the bread to a wire rack and let it cool for at least 2 hours before slicing.