Boil the kettle. In a non-stick frying or shallow pan with a lid, fry the onion, peppers, chorizo and garlic in the oil over a high heat for 3 mins. Stir in the rice and chopped tomatoes with 500ml boiling water, cover, then cook over a high heat for 12 mins.
Uncover, then stir - the rice should be almost tender. Stir in the prawns, with a splash more water if the rice is looking dry, then cook for another min until the prawns are just pink and rice tender.