Heat oil in a frying pan over medium-high heat. Add onions, and sauté, stirring occasionally, for about 5 minutes or until soft and translucent. Add curry paste, and sauté, stirring constantly, for 1 minute or until aromatic.
Add tomatoes. Sauté, stirring occasionally, for 3-4 minutes or until tomatoes break down and turn mushy. Add water to cover the tomatoes. Season with salt. Add fish. Bring to the boil. Cook, uncovered, for about 5 minutes.
Add prawns. Cook, uncovered, for another 5 minutes. Add coriander (cilantro) leaves and lemon juice. Stir gently to combine. Heat through.
Garnish with more coriander (cilantro) leaves if you like.