In a large mixing bowl, combine 1 cup of active sourdough starter and 1 1/2 cups of warm water. Whisk until the mixture is well combined and the starter is fully dissolved.
To the wet mixture, add 4 cups of all-purpose flour and 1 1/2 teaspoons of fine sea salt. Stir with a wooden spoon or spatula until a shaggy dough forms. The dough will be sticky, which is normal.
Once the dough comes together, cover the bowl with plastic wrap or a kitchen towel. Allow it to rise at room temperature for 4 to 6 hours, or until it has doubled in size.
While the dough is rising, generously grease a baking sheet with olive oil. This will prevent the focaccia from sticking and help create a crispy crust.
After the dough has risen, gently pour it onto the prepared baking sheet. With oiled hands, stretch and shape the dough into a rectangle, filling the entire sheet. It’s okay if it doesn’t reach the edges perfectly; it will expand during the second rise.
Cover the dough once again and let it rise for another 1 to 2 hours, or until it puffs up noticeably.
Preheat your oven to 425°F (220°C) about 30 minutes before baking.
After the second rise, use your fingers to dimple the surface of the dough, creating deep wells. Drizzle additional olive oil over the top and sprinkle with flaky sea salt. If you're using fresh rosemary, scatter it across the surface now.
Place the baking sheet in the preheated oven and bake for 20 to 25 minutes, or until the focaccia is golden brown and sounds hollow when tapped on the bottom.
Once baked, remove the focaccia from the oven and let it cool slightly in the pan. Transfer it to a wire rack to cool completely before slicing. Enjoy your homemade sourdough focaccia warm or at room temperature.
