Mix the Dough: In a large bowl, whisk together the sourdough starter and warm water. Add flour and salt, then mix until a sticky dough forms. Drizzle in 1 tablespoon olive oil and mix again.
Rest and Stretch: Cover with a damp towel and rest for 30 minutes. Perform 3 rounds of stretch-and-folds every 30 minutes.
Bulk Fermentation: Cover tightly and let rise for 4–6 hours at room temperature or refrigerate overnight for deeper flavor.
Prepare the Pan: Coat a 9x13-inch pan generously with olive oil. Transfer dough to the pan and gently stretch it to fit. Let rise for 2–3 hours until puffy.
Dimple the Dough: Preheat oven to 425°F (220°C). Drizzle with olive oil and use your fingertips to make dimples. Sprinkle salt and rosemary on top.
Bake: Bake for 25–30 minutes, until golden brown and crisp on the edges.
Cool and Serve: Cool slightly, slice, and serve warm or at room temperature.