Preheat oven to 200°C or 180°C fan. Heat oil in a large frying pan on medium. Pan-fry onion and garlic for 3 minutes, until soft. Add passata and parsley. Season to taste and simmer for 5 minutes.
Transfer tomato mixture to a 4-cup ovenproof dish. Stir in prawns and scatter over fetta. Bake for 15 minutes, until fetta is lightly coloured and prawns are cooked through.
Serve with salad garnish and a grinding of pepper.