Preheat the oven to 200°C.
Skin the fish and remove any bones.
Place fish in a roasting tray, season with salt, and drizzle with olive oil.
Roast fish for 8 minutes then remove to cool.
Finely chop lime leaves, deseeded chilli, ginger, garlic, and lemongrass.
Place aromatic ingredients in a bowl with mashed potato and fish sauce.
Break up the cooled fish and mix into the bowl.
Add chopped coriander and lime juice to the mixture and combine.
Form the mixture into cakes 6cm to 8cm wide.
Dredge cakes in flour and shake off excess.
Heat oil in a pan and fry cakes for 4 minutes per side.
Serve hot with lime wedges and sweet chilli sauce.