Rinse the prawns and dab dry with kitchen paper. Place in a bowl and sprinkle with half the turmeric and mix. Heat the oil in a pan on medium heat and lightly fry the prawns for around 30 seconds on each side. Scoop the prawns out and reserve.
Add the mustard seeds to the same pan. When they start to pop, add the crushed garlic and fry for a minute or two, then add the onions and salt. Sauté for a couple of minutes, until the onions have softened.
Add the remaining turmeric, chilli powder and ground coriander and cook for a minute, then pour in the coconut milk and whole green chillies. Bring the sauce up to a simmer, then put the fried prawns back in the pan.
Cover and cook for five minutes and stir in the chopped fresh coriander just before turning off the heat. Serve with rice.