In a large bowl, mix 380g filtered water with 190g active starter.
Now mix in the bread flour, and 00 flour. Mix until no dry flour remains. Cover and rest for 30–45 minutes. This resting period is called autolyse. It allows the gluten to develop naturally and hydrates the flour.
Next, add the salt, olive oil, and additional 70g water.
Use your clean hands to squish, squeeze, and fold the dough. If you have a dough mixer, this can be done in the mixer instead of by hand. Mix until the oil and water has fully incorporated into the dough (about 5 minutes).
Transfer the dough to a large bowl the has been lightly greased with olive oil. Cover the bowl and let it rest for 30 minutes at rom temperature.
Now begin your first set of stretch and folds. In a bowl, wet your hands, lift the dough from the center so the ends drape down, then tuck those ends underneath the dough. Rotate the bowl and repeat 2–4 times until the dough tightens slightly.
Let the dough rest covered for another 30 minutes with another set of stretch and folds.
Continue doing this for a total of 4 sets of stretching and folding with 30 minutes in between each time.
Preheat your oven to 450°F. Place your cast iron pan inside for 30 minutes. You want the pan to get very hot before you add the dough.
Carefully remove the hot cast iron pan from the oven and place it on the cooktop or any other heat resistant surface.
Now gently transfer the dough into your hot cast iron pan. Add a little drizzle of olive oil, and begin to poke the dough with all your fingers. You want to create many dimples
Now add an additional drizzle of olive oil, a light sprinkle of kosher salt, and dried rosemary.
Place it back into the oven and bake for 20-30 minutes or until golden and crunchy on the outside.
Let cool slightly before slicing.