Heat the oil in a large, deep frying pan. Add the onion and celery, and fry for 5 mins to soften. Add the rice and spices, and pour in the tomatoes with just under 1 can of water. Stir in the bouillon powder, pepper, garlic and thyme.
Cover the pan with a lid and simmer for 30 mins until the rice is tender and almost all the liquid has been absorbed. Stir in the prawns and parsley, cook briefly to heat through, then serve.