To make fish cakes,cut the fish into small pieces. Place the capsicum,chillies,coriander,spring onions,garlic,lemongrass and fish sauce into a food processor and blend to a paste.
Add the fish pieces to the paste and blend well. Mix to a smooth paste,adding coconut milk and egg. Place the mixture in a bowl and chill in refrigerator for at least two hours.
Half-fill a heavy-based frying pan with oil and warm over a medium to high heat.
Shape the fish mixture into small patties. Carefully lower into the hot oil in batches. Fry until crisp and brown,turning once during cooking.
Remove from the oil using a slotted spoon and set aside to drain on kitchen paper. Repeat the process with the remaining fish cakes.
For the salad,cut the cucumber into thin slices. Dissolve the sugar in the vinegar and toss the cucumber slices in it. Add chilli,shallots and coriander. Sprinkle with peanuts and deep-fried shallots just before serving.
Serve fish cakes with cucumber salad.