To make the prawn marinade, mix the prawns and the remaining marinade ingredients in a bowl and leave to marinate for a few hours.
To make the prawns, heat the oil and fry the curry leaves and spices for 1 minute. Add the onion and garlic and sweat until softened. Add the tomatoes, tomato purée and chillies and simmer for 20 minutes. Add the marinated prawns, stir through and simmer gently for 5 minutes until cooked through. Taste and season with salt and pepper.
Preheat a deep fat fryer to 180C. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
To make the dough, Mix the dry ingredients together in a bowl and pour in the oil and 5–6 tablespoons water. Mix to a firm dough and roll into small, round balls – about the size of a golf ball. Grease a work surface with some oil and roll out each ball to the thickness of a £1 coin. Place each disc in the fryer until puffed and lightly golden all over. Carefully remove with tongs and leave to drain on kitchen paper.
Spoon the prawn mixture on top of each puri. Garnish with the coriander and serve.