Preheat the oven to 220C/425F/Gas 7. Place the cherry tomatoes in a roasting tin and sprinkle over the red onion, garlic and olive oil. Roast for 20 minutes until the tomatoes are softened.
Stir in the rice, chicken, chorizo, rosemary, chicken stock, saffron and some salt and pepper, mixing well together. Return to the oven for 20 minutes.
Stir in the prawns and return to the oven for a further 10 minutes until the rice is tender and the chicken is cooked through.