Heat the oil in a large frying pan on medium high heat. Add the onion, turmeric, chilli powder, garam masala, salt and green chillies and stir till onions turn crispy brown, about 3 minutes.
Add the prawns to the frying pan, stirring well, and cook for another 5 minutes. Add the water and stir to release any browned bits from the bottom of the pan. Add the vegetables and mix well, then cook covered for 3 minutes. Add coriander and cook covered for a further 1 to 2 minutes, until vegetables are tender and prawns are opaque.
Serve on a platter with the chopped cherry tomatoes on top for garnish. Best eaten with boiled rice or naan.