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Tip the rice, onion and half the butter into a large microwave-proof bowl. Cover and cook in the microwave on High for 3 mins. Stir in the stock and passata, then continue to cook, uncovered, for 10 mins. Give it a good stir and mix in the tomatoes and mozzarella. Microwave on High for a further 8 mins until the rice is cooked and the tomatoes have softened.
Leave the risotto to relax for a few mins, then stir in the remaining butter, parmesan and basil. Season to taste and serve straight from the bowl.