Mix up the leaven in a 3-4 qt bowl the night before.
First add 30g of starter then 100g water followed by 100g flour.
Mix with a sourdough whisk, spatula or spoon until combined. Cover and leave out at room temperature overnight until it has lots of bubbles or has doubled in size.
For the Dough
The next morning add 618g of water to the leaven followed by the 910g of unbleached flour and mix with a spoon or spatula until flour is incorporated.
Cover and rest dough for 30 mins and up to an hour. This is the autolyse.
Add the remaining 65g of water and the heaping tablespoon of salt to the dough squeezing it into the dough with your hands until incorporated.
After 30 mins begin your first of 5 stretch and fold sequences waiting 30 mins between each.
After the 5th set wait 30 more mins and then divide the dough into 2 equal loaves and shape.
Add shaped dough to floured banneton baskets or alternative.
Place bannetons into a sealed plastic bag or container and into the refrigerator overnight or up to 3 days. If baking same day then let rest at room temperature until puffy and billowy.
Baking
Preheat oven and dutch oven(if using) to 475 °F for about 30 mins.
Remove first loaf from the fridge and place your silicone sling on top then flip dough out onto it. Score dough with a lame or sharp knife.
Place the dough and sling into the dutch oven or alternatively a bread pan with another stacked on top to trap steam.
Bake for 25 mins with the lid on and then 7 mins with the lid off. Lid of time is for browning so you can adjust the times based on your preference.
Check doneness by knocking with your knuckle on one of the darker spots of the bottom of your loaf to check for a hallow sound. Alternatively and for most accuracy check the internal temperature to reach 205-210 °F.
Remove loaves from dutch oven or bread pan immediately and cool on a wire rack for at least 4 hours before storing.
Notes
I have used both active and cold sourdough starter for this recipe. The more active the starter, the better the rise and lighter crumb. The older the starter can result in a more dense crumb or even gummy if it's too hungry. Everyone's starter is a bit different so you may need to feed yours a few times before using it depending on the result you desire.
The AP flour I use is 11.5% protein content
I don't use my tap water in my starter or dough because of the chemicals its treated with in my city that interfere with the microorganisms.
I store my bread that I want to have out for eating in a zip tight bag or to maintain a semi crisp crust I wrap it in parchment paper or a paper bag for the first day only. After the first day I transfer to a zip tight bag.