Heat the oil in a large saucepan over moderate heat. Add the spring onions and garlic and fry for 1 minute or until soft. Add the celery and capsicums and cook for 5 minutes or until the capsicums are crisp-tender. Stir in the sausage.
Add the rice, stirring to coat. Add the stock, 420ml water, the thyme and black pepper, and bring to the boil. Reduce to a simmer, cover and cook for 17 minutes or until the rice is tender. Stir in the prawns, cover and cook for 3 to 4 minutes or until the prawns are firm and pink.