Bring a large pot of lightly salted water to a boil. Add the pasta and cook for 8 minutes, or until tender. Drain.
In the container of a food processor combine the garlic, onion and oregano. Pulse until finely chopped. Heat the olive oil in a large frypan over medium heat. Add the onion mixture; cook and stir until fragrant and almost golden. Mix in the tomatoes, basil, salt and pepper. Simmer for about 5 minutes while the pasta is cooking, stirring occasionally.
Mix in spinach until it wilts, then just before the pasta is done stir in the prawns. Cook until heated through. Toss with pasta in a large serving bowl and mix in mozzarella cheese.