In a large heavy ceramic or glass mixing bowl, whisk together 500g bread flour and 350g of the warm filtered water (28°C / 82°F) until no dry patches remain. Do not add salt or levain yet. Cover the bowl securely with a damp kitchen towel or bowl cover and let rest at room temperature for 45 minutes to initiate enzymatic starch breakdown and gluten strand development.

Uncover the autolysed dough. Spread 100g active mature sourdough starter across the surface and dimple it into the matrix using wet fingertips. Dissolve the 10g fine sea salt in the remaining 25g reserved warm water, then pour over the dough. Perform slap-and-fold techniques in the bowl for 4 minutes until the wet mixture is cohesive and begins to hold structure. Cover and rest for 30 minutes.

Over the next 2 hours, perform 4 sets of stretch-and-folds spaced 30 minutes apart. With wet hands, scoop under one quadrant of the dough, stretch it upward gently without tearing, and fold it across the center. Rotate the bowl 90 degrees and repeat for all four corners. After the final fold, leave undisturbed at 24°C (75°F) for an additional 2 hours until the dough exhibits domed edges, aerated bubbles, and 40% volume expansion.

Lightly dust your work surface with rice flour. Gently tip the aerated dough out, taking care not to deflate the delicate internal gas bubbles. Using a bench knife held at a 45-degree angle, gather the dough into a loose, taut sphere by pulling it toward yourself across the un-floured marble. Allow the dough to rest uncovered on the bench for 20 minutes to relax gluten tension.

Dust an oval or round cane banneton generously with a 50/50 mixture of rice flour and bread flour. Flip the relaxed dough onto its top side. Fold the bottom third up, the left and right sides toward the center like an envelope, and roll from top to bottom into a tight cylinder. Seal the seam and transfer seam-side-up into the prepared banneton. Slide into an airtight plastic bag and refrigerate at 3°C to 4°C (38°F) for 14 to 16 hours for cold enzymatic flavor development.

Place a heavy 5-quart cast iron Dutch oven with lid on the middle oven rack. Preheat oven to 250°C (485°F) for a full 60 minutes. Invert cold dough onto parchment paper. Dust surface with rice flour and score decisively with a curved baker's lame at a 30-degree angle, 1/2-inch deep along the spine. Carefully lower into the blazing Dutch oven, drop 2 ice cubes beneath the parchment for intense steam, seal the lid immediately, and bake covered for 20 minutes.

Carefully remove the Dutch oven lid. Reduce oven temperature to 220°C (430°F). Bake uncovered for 20 to 22 minutes until the crust turns deeply mahogany, the blistered ear is dark caramel, and tapping the base yields a hollow resonance. Transfer immediately to a wire cooling rack and wait at least 2 full hours before slicing to prevent the interior crumb from gummy collapsing.