Place the prawns in a bowl, add the lime zest and mix well.
Cover with cling film and refrigerate.
Prepare the chillies. Use gloves if you can and be careful not to touch your face. Cut off the stalk, then cut in half lengthways. Remove the seeds and pith (white part) with a sharp knife. Cut each half into very thin long strips. Add the strips to the prawns and mix well again. Cover and refrigerate.
Combine the mayonnaise with the fish sauce, sliced spring onions and wasabi paste. Taste and adjust seasoning if necessary.
To serve: pile the prawns on a serving platter or dish and serve the mayonnaise in a separate dish.
Garnish with quartered limes.