Blend pepper, chillies, coriander, spring onions, garlic, lemongrass, and fish sauce into a paste using a food processor.
Add fish pieces to the paste and process, then mix in coconut milk and egg to form a smooth paste.
Stir in the 3mm sliced green beans and chill the mixture for at least two hours or overnight.
Heat oil in a heavy-based pan and shape the paste into small patties.
Fry patties in batches until brown and crisp, turning once during cooking.
Drain cooked fishcakes on kitchen paper and repeat with remaining mixture.
Slice cucumber into 5mm rounds for the salad.
Dissolve sugar in vinegar and toss the cucumber slices in the liquid.
Fold in the chilli, shallot, and coriander.
Top with peanuts and fish sauce immediately before serving.