1.Shell and devein prawns, leaving tails intact. Cut along back of prawn, taking care not to cut all the way through; flatten prawn slightly.
2.Rub turmeric and chilli into prawns in medium bowl.
3.Heat oil in large frying pan; cook mustard seeds and garlic, stirring, until seeds start to pop. Add prawn mixture; cook, stirring, until prawns just change colour. Stir in onion and coriander.