Heat the oil in a large heavy bottomed pan over a moderately high heat for 3 minutes. Add the mustard seeds and when they have popped and the garlic and onions. Cook for about six minutes, stirring from time to time.
Add the ginger, chillies and cumin and cook for a minute more, then add the coconut milk and salt. Bring to the boil, lower the heat and simmer, partially covered, for 8 minutes.
Add the prawns and their juices and continue cooking, uncovered, until they are just heated through.
Transfer to a serving dish and garnish with coriander. Serve with a Semolina Pilaf and a side dish