1. In a large mixing bowl, combine the active sourdough starter and warm water.
2. Gradually add the flour and sea salt to the mixture, stirring until a shaggy dough forms.
3. Add the olive oil and mix until incorporated. The dough will be sticky.
4. Cover the bowl with a damp cloth and let it rest at room temperature for about 4-6 hours, or until doubled in size.
5. Generously grease a baking sheet with olive oil.
6. Pour the risen dough onto the baking sheet and stretch it gently to fit the pan. Cover it again and let it rise for another 1-2 hours.
7. Preheat your oven to 425°F (220°C).
8. Dimple the surface of the dough using your fingers, drizzle with more olive oil, sprinkle with flaky sea salt and rosemary.
9. Bake for 25-30 minutes or until golden brown.
10. Let it cool slightly before slicing and serving.