Bring a pan of salted water to the boil and tip in the sliced potatoes. Boil for 4-5 mins until just tender, then drain and leave to steam-dry. Heat the oven to 240C/220C fan/gas 9 with two large baking sheets inside. Gently shape the dough into two rectangles, about 30 x 20cm, on a lightly floured surface using your hands. Carefully transfer to sheets of baking parchment. Evenly scatter over the mozzarella and parmesan, then top with the sliced potatoes, rosemary leaves and red onion. Drizzle over the 2 tbsp olive oil and carefully transfer one to a heated baking sheet. Bake one at a time on the highest oven shelf for 12-18 mins until golden and bubbling. Drizzle over a little more olive or chilli oil, and season with black pepper. Serve with a green salad, if you like.
