Mix the Dough: Combine flour, water, and sourdough starter in a bowl. Cover and rest 30 minutes.
Add Salt & Oil: Mix in salt and olive oil, knead gently until smooth and elastic. Perform stretch-and-fold every 30 minutes for 2 hours.
Bulk Fermentation: Cover and let dough rise 2-3 hours until doubled.
Shape Dough: Grease baking pan, transfer dough, and press to fit. Dimple surface with fingers. Drizzle olive oil, sprinkle rosemary and salt. Final Proof: Rest 30-45 minutes until slightly puffy. Preheat oven to 220°C (425°F).
Bake: Bake 20-25 minutes until golden brown and crisp. Cool slightly before slicing.