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In stand mixer bowl, mix ripe starter and water briefly with a spoon.
Add white flour and whole wheat flour; mix in mixer until no more dry clumps of flour, cover, and let stand for 30 minutes.
Mix water and salt in a bowl and add to dough in mixer bowl; mix for 5 minutes.
Transfer to proofing bowl and start timer for 30 minutes. Stretch and fold every 30 minutes for 3 hours.
Split into two equal sized balls and pre-shape. Let stand for 30 minutes.
Final shape and transfer to proofing baskets. Cool in fridge for 30 minutes to an hour (or overnight).
Preheat oven to 500 degrees with Dutch oven inside.
Score bread.
Place proofed dough in Dutch oven. Lower temperature to 450 degrees.
Bake for 25 minutes covered and 18 minutes uncovered.