Cook the potato in an 18cm or 7" saucepan of well-salted water until soft; this should take 10 minutes, then cool and mash. I use 1 teaspoon of salt in a pan this size.
Slice the chilli pepper as finely as you can and remove the seeds if you want.
Slice the spring onion as finely as you can.
Grate the ginger.
Remove the tough stem from the centre of the lime leaves and slice them as finely as you can.
Mash the potatoes and add the remaining ingredients for the fish cakes.Form into balls a little smaller than the size of golf balls, then lightly press these balls to form a small tuna patty.
Heat 5mm-1cm (¼-½") of oil in a 30cm or 12" frying pan over a medium-high heat and fry the cakes until crisp and golden.Drain on a kitchen towel and allow to cool for 3-5 minutes before eating.
For the Dipping Sauce:
Slice the chilli pepper and spring onion as finely as you can.
Mix together all of the ingredients for the dipping sauce.