Put the sourdough starter and the tepid water into a large bowl. Mix together with a spatula. Tip in the flour and mix until well combined. Cover and leave for 30 minutes.
Sprinkle with 1.5 tbsps of the salt. Wet your hand, then use it to work the salt into the dough, pulling and scrunching. When combined, cover and put somewhere reasonably warm (I use my airing cupboard) for 30 minutes.
Uncover the dough and pull one side out, then fold it over itself. Work around the dough, pulling each side out and folding over. Cover and leave for another 30 minutes. Repeat twice more (so that you have folded 3 times). Cover and leave for another two hours.
Put 2 tbsps of the olive oil into a 20cm x 30 cm ovenproof dish. Tip the dough into the dish and move it around, folding and pulling the dough until the oil is incorporated. Cover and leave in a warm place for 3 hours.
Preheat the oven for 220 degrees Celsius. Push your fingertips into the dough to make dips. Sprinkle over the remaining salt and 2 tbsps olive oil. Stick the rosemary sprigs into the dough. Put in the oven for 30-40 minutes, until golden.
Remove from the oven and drizzle over the remaining olive oil. Carefully remove from the dish and leave to cool before eating.