Heat the oil in a large frying pan, add the onion and cook, stirring, over a medium heat for about 5 minutes until softened. Add the chilli and curry paste then cook for 1–2 minutes, taking care not to allow the paste to burn on the base of the pan.
Pour the coconut milk into the pan and stir well Add the diced tomatoes and stir again. Cook over a low–medium heat for about 10 minutes or until the tomatoes have softened and the sauce has thickened slightly, stirring occasionally.
Stir the mango into the curry and cook for 1–2 minutes. Add the prawns then simmer gently for about 5 minutes until the prawns are pink, tender and cooked through. Stir in the chopped coriander then serve immediately.