Heat the oil in a large nonstick frying pan over moderate heat. Add the onion and garlic and cook for 5 minutes or until the onion is soft. Add the capsicum and cook for 4 minutes or until crisp-tender.
Add the tomatoes, the stock, olives, fennel seeds, and a pinch each of cayenne pepper and salt. Bring to the boil, then reduce to a simmer. Add the snapper and morwong, cover and cook for 7 minutes or until the fish is cooked. Stir in the basil just before serving.