Heat the oven to 240°C/fan220°C/gas 9. Heat a 20cm heavy-based, ovenproof frying pan on the hob. Add 2 tbsp extra-virgin olive oil and, when it shimmers, pour in half the batter. It should sizzle like Yorkshire pudding batter. Once the edge is set, turn the heat down a bit. When the mixture begins to bubble, scatter over half the rosemary leaves, then continue to cook on the hob until the sides begin to turn golden.