Place your large mixing bowl on an electronic scale, and weigh 300g of warm water (~35-38C)
Using a silicone spatula, scoop out all the leaven and pour into the water. Using a bell whisk, thorough combine the water and leaven.
Pour in 500g strong unbleached break flour, and mix thoroughly with a silicone spatula. Then, transfer to a plastic dough scraper, and scrape all the dough from the sides of the bowl and combine into a scraggy ball of dough. Cover with a warm damp tea towel and leave to autolyse at room temperature for an hour.
Weigh 15g warm water, and 10g fine sea salt and stir thoroughly. If you over weigh 15g water (easily done), use a pipette to extract the excess. Using the dough scraper, detach the dough from the base of the bowl and turn to form into a round.
Spoon half the saline mixture over the top of the dough, then poke several times with your fingers (as if making a Focaccia). Turn the dough over, spoon over the rest of the saline solution, and poke again several times.
Using the dough scraper, form the dough into a ball and create tension in its surface. Cover with a damp tea towel and leave at room temperature. Wait 10 minutes.
Stretch and Fold #1: With the tips of your fingers (moistened with water), lift the dough from the corner opposite you, stretch, and fold over. Rotate the bowl 90 degrees and repeat the stretch and fold three more times. Use the dough scraper to create tension and the boule shape, cover with tea towel and leave for 30 minutes.
Stretch and Fold #2: With moistened fingertips, lift the dough from the corner opposite you, stretch, and fold over. Rotate the bowl 90 degrees and repeat the stretch and fold three more times. Use the dough scraper to create tension and the boule shape, cover with tea towel and leave for 30 minutes.
Stretch and Fold #3: With moistened fingertips, lift the dough from the corner opposite you, stretch, and fold over. Rotate the bowl 90 degrees and repeat the stretch and fold three more times. Use the dough scraper to create tension and the boule shape, cover with tea towel and leave for 30 minutes.
Stretch and Fold #4: With moistened fingertips, lift the dough from the corner opposite you, stretch, and fold over. Rotate the bowl 90 degrees and repeat the stretch and fold three more times. Use the dough scraper to create tension and the boule shape, cover with tea towel and leave for 15 minutes.
Gently flour the inside of the banneton proving basket. Create tension one last time with the dough scraper, then scoop up the entire dough boule and turn it into the floured banneton basket (so that the top is now at the bottom of the basket). Sprinkle a thin layer of flour over the dough, and flatten the top and shape the edges by patting down with your fingers and palms. Cover with the cloth liner and refrigerate for 4-5 hours.