In a large mixing bowl, combine the sourdough starter, salt, and water using a spatula until just mixed. Incorporate the flour and stir again until no dry spots remain.

If feasible, perform a single stretch and fold after 30 minutes: reach into the bowl, pull the dough up, and fold it into the center. Rotate the bowl slightly and repeat 8 to 10 times.

Drizzle the surface of the dough with a small amount of olive oil, spreading to coat. Cover the bowl and leave it to ferment at room temperature for 4 to 18 hours, depending on ambient temperature and starter vitality. Look for the dough to double in volume rather than follow a strict timetable. A straight-sided container is especially useful for tracking this rise.

When the dough has nearly doubled, pour 2 tablespoons of olive oil into a 9×13-inch baking pan. If using glass, consider greasing with butter as an added precaution against sticking.

Gently deflate the dough and transfer it into the prepared pan, placing it in the oil's center. Fold it from each side inward to shape a rough rectangle, then flip it seam-side down. Drizzle with 1 tablespoon olive oil and leave uncovered for a final rise of 4 to 6 hours until puffy and nearly doubled again.

Preheat your oven to 425ºF. Lightly oil your fingers and press them into the dough across the surface to create dimples and stretch the dough to fill the pan. Liberally sprinkle with flaky sea salt.

Bake for approximately 25 minutes or until the top and edges are richly golden. Remove from the oven, lift the focaccia from the pan, and let it cool on a wire rack for at least 20 minutes before slicing.