In a small bowl mix together the flour (flours if making your own bread dough) and salt.
Put 200 grams of the warm water in the bowl of a stand mixer.
Add the yeast. If using instant yeast, continue to the next step. If using active yeast, mix into the water with a fork then let sit for 10-15 minutes until bubbly.
Add the starter and mix to incorporate.
Add the olive oil.
Add the flour and salt mixture.
Using the dough hook, mix on low to combine.
Start adding the additional 100ml of water a bit at a time and increase the speed of the mixer to medium. Increase to medium-high once all the water is added and continue mixing -- 15 to 20 minutes -- until all of the water is absorbed.
Lightly oil the square tub and put the dough into it. Cover and let rest for 2-3 hours until doubled or tripled in bulk. (See video below.)
Dust your work surface with all-purpose flour and some semolina if you have it.
Prepare two baking sheets with parchment paper. Dust with flour and semolina.
Remove the lid from your tub and invert above your work surface. Be patient, but the dough should come out easily and form a square blob roughly 12" x 12" -- nudge to get an even square if needed but don't knock the air out of your dough.
Cut the square in half. Then cut each half again in half lengthwise. You should have 4 strips. Cut each of these into three equal pieces. With floured hands gently form each piece briefly to form a rectangle. Place 6 per sheet pan, spacing evenly.
Loosely cover with oiled plastic wrap and let rise until puffy, maybe another 30-45 minutes.
Heat oven to 425°F / 220°C.
Remove the plastic wrap. If desired, dimple the tops with your fingertips, and spray with warm water immediately before popping into the oven. The water will give them a bit more crust.
Bake for 15 minutes. Turn around and, if baking both at once, change racks. Bake another 5-10 minutes or until golden brown.
Remove from the oven and let rest on the sheet pans for 5-10 minutes before removing to cooling racks.