In a large bowl, mix sourdough starter with water. Add flour and salt. Mix until all the flour is combined.

Let the dough rest for 30 minutes. After resting, perform a series of stretch and folds. Grab a section of the dough, stretch it upwards, and fold it over the rest. Rotate the bowl and repeat until you’ve done it four times. Repeat this process every 30 minutes for 2 hours. Cover the bowl and let dough rise at room temperature for about 4-6 hours, or until it has doubled in size.

Drizzle a generous amount of olive oil into a 9x13 inch baking dish. Add the dough and gently press it out so it fills the dish. Cover and let the dough proof for another 2-4 hours at room temperature, until it looks puffy.

Preheat your oven to 450F/232C. Dimple the dough with your fingers. Drizzle olive oil over the top, sprinkle with fresh rosemary, and a generous amount of flaky sea salt.

Bake for 20-25 minutes, or until the focaccia is golden brown on top. Remove from the oven and let it cool in the pan for a few minutes before transferring to a wire rack.

Serve with oil for dipping or make your favourite sandwich.