Mix the flour, warm water, sourdough starter, and salt in a bowl or stand mixer until everything is evenly moistened.

Knead briefly until the dough starts to form, then let it rest for about 10 minutes.

Slowly add the olive oil while mixing until the dough absorbs it fully and becomes smooth and elastic.

Transfer the dough to a lightly oiled bowl, cover, and let it rise for about 2 hours.

During the rise, perform a gentle stretch-and-fold every 30 minutes to strengthen the dough.

Coat a 9×13-inch baking pan generously with olive oil.

Turn the dough into the pan, keeping as much air in it as possible.

Over the next hour, gently stretch the dough toward the edges every 30 minutes until it fills the pan.

Cover and allow the dough to rise until puffy and airy.

Wet your fingertips and press dimples across the entire surface of the dough.

Drizzle with olive oil and scatter fresh rosemary and flaky sea salt on top.

Preheat the oven to 425°F (220°C).

Bake on the lower rack for about 30 minutes, or until the focaccia is golden brown and crisp along the bottom.

Let the bread cool briefly before removing it from the pan and slicing.