Cut the prawns into 1 cm pieces. Heat the oil for deep frying in a large heavy based saucepan or wok to 180-190 C or until a cube of bread browns in 30 seconds.
Add the prawns and deep fry until lightly golden, about 90 seconds. Remove with a slotted spoon and set aside to drain on kitchen paper.
Heat 2 tbsps oil in a large frying pan or wok. Add the mustard seeds and when they start to pop add the curry leaves and the sliced shallots. Cook, stirring, for 5 minutes or until the shallots are soft.
Stir in the turmeric, chilli powder, green chilli and some salt, Cook, stirring, for a further 2 minutes.
Add the prawns and stir fry over a medium low heat for 5 minutes. Pour in the lemon juice , then stir in some black pepper. Remove the pan from the heat and serve straight away.