Put the prawns, spring onions, lime rind, coriander and fish sauce in a food processor or blender and process for about 20 seconds until well combined but retaining a little texture.
Shape the mixture into 12 balls and lightly dust them in cornflour. Heat a little oil in a nonstick frying pan and fry the balls for 2–3 minutes until golden all over. Drain on kitchen paper.
Meanwhile, make the chilli sauce. Put the vinegar and sugar in a small saucepan and heat gently to dissolve the sugar. Increase the heat and boil for a couple of minutes until the mixture becomes syrupy. Add the chilli and red onion and allow to cool, then stir through the coriander.
Serve the prawn balls with the chilli sauce in a bowl for dipping, and a crisp green salad, if liked.