Heat oil in a large frying pan on medium. Cook onion for 3 minutes, stirring, until soft. Add garlic and chilli and cook for 1 minute, until fragrant. Add tomato and 1 cup water and bring to boil. Season well, cover, reduce heat to low and simmer for 10 minutes.
Add prawns and simmer for 5 minutes, covered, until prawns are opaque. Remove from heat and stir through fetta and herbs. Stand covered for 5 minutes.