1 cup fed and active sourdough starter
OR
Levain: 1T active sourdough starter, 75g flour, 75g water
300-500 grams filtered water
700 grams all-purpose or bread flour (KAF preferred, that's the good stuff)
1/4-1/2 teaspoon baking soda (cutting sourness)
1 tablespoon salt
Optional ingredients (nuts, herbs, fruit, etc)
olive oil (to grease bread loaf pan)
water (for steam)
Feed starter 4-8 hours before mixing. Depends on how active the starter is.
OR
Levain: Mix levain and leave in the oven with the oven light on for 10-11 hours, overnight.

Combine all ingredients except salt. Add or subtract water based on overall stickiness. Dough should be slightly sticky but not trying to fuse with your hand. Let sit for 30 minutes (autolyse).

After autolyse, mix in salt. Commence with stretch and fold. Every 30 minutes, stretch and fold dough, 6 times (2.5 hours). At fold 3, you can mix in any optional ingredients.

After the last stretch and fold, let rise in bowl for 1 hour.

Weigh dough, then divide in half equally by weight. Shape loosely and allow to rest for 20-30 minutes, then do the final shaping. Make sure the outer layer is tight but not torn.

Place to rise in well-floured tea towels in bread loaf pans for 4 hours on bread proof setting (~100 degrees F).

Take loaves out of oven, place cake pan inside on lower rack and preheat oven to 450. Boil water while waiting.

Score loaves fairly deeply with a sharp knife or lame. Place loaves in oven and pour boiling water into the cake pan beneath (beware steam).

Bake loaves for 20 minutes.
Reduce temp to 400 and bake for another 10 minutes.
Remove cake pan full of water from oven (carefully) and bake for another 15 minutes, until crusty and very browned.

Turn out on cooling racks and allow to cool for at least 3 hours.