Combine the sourdough starter, water, and salt. Then, mix in the flour until a sticky dough forms. Let it rest for 30 minutes.
Stretch and fold the dough over itself, rotating the bowl as you do so. Perform about 6 stretches and folds. Generously drizzle oil over the dough and coat it thoroughly. Cover and let it rise at room temperature overnight for about 8-12 hours.
The next day, the focaccia dough should have doubled in size and appear somewhat bubbly. Generously coat a cast-iron skillet with olive oil. Flip the proofing bowl containing the focaccia dough upside down and shake gently until the dough releases into the cast-iron skillet.
Lightly oil your fingers and gently pat the dough down, then fold the four sides of the dough into itself (like closing the flaps of a cardboard box). Turn the dough over so it is seam side down, generously drizzle with olive oil, and coat thoroughly. Let it rise for another 4-6 hours in the oven turned off. The dough should double in size and spread to fit most of the pan.
Take the proofed focaccia dough out of the oven and preheat the oven to 425°F.
Oil your fingers and gently dimple and stretch the dough to fit the pan. Sprinkle generously with flaky salt or a seasoning salt & herb mix of your choice.
Bake for 25 minutes or until golden brown.