Preheat the oven to 180C/360F/Gas 4.
Place the pollack fillet on a baking tray, dot with the butter and bake in the oven for 10-15 minutes. Remove the fish from the oven and leave to cool.
Once cool, flake the fish.
Bring a pan of salted water to the boil and blanch the prawns for 30 seconds. Drain the prawns and allow to cool. Cut the prawns into small pieces.
Boil the potatoes in a pan of boiling salted water until tender, drain and mash.
In a large bowl, mix together the flaked pollack, prawns, mashed potatoes, spring onions, red chillies, fish sauce, lime zest and juice and the curry powder, coriander, salt and freshly ground black pepper.
Using your hands, shape the mixture into fishcakes, place on a tray or large plate, cover with cling film and chill in in the refrigerator for 30 minutes.
Heat some vegetable oil in a frying pan until hot and shallow-fry the fishcakes, in batches, for 4-5 minutes on each side, or until golden-brown. Place the cooked fishcakes on a plate lined with kitchen paper to absorb excess oil.
For the pineapple salsa, mix together all the salsa ingredients in a bowl.
Serve the cooked fishcakes with the salsa and a drizzle of chilli sauce.