Add sourdough starter to a large bowl and place it on a kitchen scale set to grams. Tare scale to 0.
Add warm water until scale reaches 300 grams and combine water and starter with a wooden spoon. Tare scale to 0.
Add flour until scale reaches 500 grams, then add salt on top.
Combine ingredients with wooden spoon until a very sticky dough forms.
Cover bowl with a plate and lest rest for 20 minutes.
After 20 minutes, uncover the bowl and wet hands. Stretch and fold the dough about 10 times by pulling up one side and pushing down on the other side, going around the ball of dough.
Cover the bowl with a plate and let rest for 20 minutes.
Repeat stretch and fold process 2 more times at 20 minute intervals.
Cover the bowl with a plate or plastic wrap and let the dough rise at room temperature for 4 or 5 hours.
After the dough has risen for the first time, generously flour a work surface and turn the dough out onto it.
Generously flour the top of the dough, and push down into a 10 x 6 inch rectangle.
Roll each side of the long sides of the rectangle to the center and push down, then flip the dough over, seam side down and tuck the ends in underneath.
Generously flour a large linen cloth and carefully transfer dough onto it.
Dust the top of the dough with flour and cover with the cloth.
Let rest for 60 minutes for a second rise.
Preheat a large, flat cast iron skillet or pizza stone in center rack of the oven to 450 degrees Fahrenheit.
Uncover dough and carefully transfer to preheated pan.
Bake for 30 to 35 minutes, until slightly golden brown.
Once baked, wrap in a clean cloth and let cool for at least 1 hour before slicing. The bread bakes more after it's out of the oven!
This bottom crust can be a little hard, so wrapping it in a cloth while it cools helps to soften it.
After resting for at least 1 hour after baking, enjoy!