Put the sugar and vinegar into a small saucepan and bring to the boil. Peel and finely dice the onion then add to the vinegar and continue cooking until the onion is tender and translucent.
Finely chop the chillies and add to the pan, together with the ginger. Continue cooking for a couple of minutes then remove from the heat. Squeeze the limes and stir the juice into the syrup. Finely chop the mint leaves, stir in and set aside.
Finely chop the prawns, either by hand or in a food processor. Take care not to over-process. Chop the coriander and fold in.
Once the peas are defrosted, roughly chop or blitz briefly in a food processor then fold them into the prawns and coriander, season lightly with both salt and black pepper.
Shape the mixture into 6 balls then flatten them in the palm of your hand and set aside on a tray and refrigerate for 30 minutes.
Warm the groundnut oil in a shallow pan. Lower the cakes into the hot oil, lightly frying for 4 minutes on one side, then 4 minutes on the next, until the mixture is pale gold. Lift out and serve with the chilli mint dip.