Tip your tomatoes into a small pan and add your peeled onion, drizzle of olive oil and some salt and pepper. Bring to a boil, then pop on a lid and simmer for a while until thick. Remove onion, add your basil and check seasoning.
Unroll your pastry and spread your tomato sauce over the top. Arrange your mozzarella and pepperoni on top, then bake for 15-20 mins until your pastry is golden and your cheese has melted.