In a large bowl, mix flour, water, salt, and sourdough starter. Stir until just combined. Cover and let rest for 30 minutes
After the 30 minutes is up, perform 4 sets of stretch and folds every 30 minutes, making sure to cover the bowl with a bowl cover or damp cloth. To perform stretch and folds, wet hands then grab one side of the dough and stretch it up and over to the opposite side. Rotate the bowl a quarter turn and repeat this process for all four sides. (SEE TIPS)
After you’ve completed the stretch and folds, it’s time for bulk fermentation. Cover the bowl again and let the dough rise at room temperature until it has increased in size by about 75%. This usually takes about 4 to 6 hours, depending on the temperature of your kitchen. For best results, you can also refrigerate the dough for 12 to 24 hours after the initial rise. This slow fermentation enhances the flavor and texture of your sourdough ciabatta dough.
When your dough has finished fermenting, it’s time to shape it. Turn the dough onto a lightly floured surface. Be careful not to deflate it too much. Gently shape the dough into a rectangle. Using a bench scraper, divide the dough into two, then divide each half into four equal pieces to form 8 small rectangles. Then, lightly shape each piece into a rectangle.
Place the shaped dough on a sheet pan lined with a piece of parchment paper, cover it with a tea towel or plastic wrap, and let it rise for 1 to 2 hours in a warm place, while the oven preheats.
Preheat to 450 degrees. Bake for 20–25 minutes, or until golden brown and crusty.
Let cool on a wire rack at least 1 hour before slicing.