In a large bowl, combine the flour and water. Mix until no dry bits remain. Cover the bowl and let the dough rest while the starter becomes active.

Once the starter has doubled in size and is bubbly, add it to the dough. Wet your hands and spread the starter over the surface of the dough, then gently knead or fold for 2 minutes until fully incorporated. Cover and let rest for 30 minutes.

Sprinkle the salt evenly over the dough. Use your fingertips to gently press it in, then knead or fold again for 2 minutes to distribute. Cover and rest for another 30 minutes.

To perform a coil fold, wet your hands and loosen the dough from the sides of the bowl. Lift it from the center and fold it under itself. Rotate the bowl and repeat on the other side. Cover and rest for 30 minutes.

Repeat the coil fold and 30-minute rest two more times.

After the final fold, cover the dough and let it rest for 90 minutes at room temperature. Then transfer to the refrigerator and rest overnight (8 to 12 hours).

The next day, remove the dough from the fridge and coat the surface with olive oil. Generously oil a cold Dutch oven and transfer the dough into it. Gently stretch the dough toward the edges by dimpling it with your fingertips.

Cover and let the dough rest at room temperature for 2 hours, until it looks light, puffy, and bubbly.

Preheat the oven to 425°F (220°C).

Scatter the garlic, rosemary, and flaky salt (if using) over the surface of the dough. Drizzle with extra olive oil and gently press the toppings into the surface with your fingertips.

Bake for 25 to 30 minutes, or until golden brown on top and crisp around the edges.

Remove the focaccia from the Dutch oven and let it cool on a wire rack for 20 minutes before slicing and serving.