To make the Levine, mix together 74 g all-purpose flour, 37 g fed sourdough, and 74 g water. Let this sit in proofing box, oven on proof, or a warm spot for 6 hours.
An hour before the Levine is ready, mix together 850 g All-Purpose Flour, 550 g water, and 15 g salt. Let sit for an hour in the same warm spot as the Levine.
When the Levine is ready, create a well in the bread flour mixture and pour the Levine into it. Mix to combine. This can be done by hand or in a KitchenAid mixture with a dough hook. Once fully combined, let rest for 15 minutes.
Over the next hour and a half, stretch and fold the dough. To stretch and fold- take one side of the dough, pull it as far as it will stretch then fold it to the other side of the dough. Do this all around the dough.
After the last stretch and fold, left the dough rest 3-6 hours until just under double in size
Turn dough out onto the counter and divide into two loaves. Shape dough into two balls creating surface tension across the top.
Place into proofing basket, top side down, and place into a proofing bag before putting into the refrigerator overnight for 12-24 hours.