For the fish balls, grind the fish pieces in a blender until they have the consistency of minced meat. Mix in the curry paste briefly.
Add the egg and continue to mix until a homogeneous mass is obtained. Mix this thoroughly in a bowl with the spring onions, kaffir lime leaves, coriander, sugar and salt. (Alternatively, finely chop the meat with a knife and mix in the remaining ingredients).
Shape the mixture into balls about 2 cm in diameter and let set in the refrigerator for about 20 minutes. Fill a narrow, high saucepan 5 cm high with oil and heat this to 190°C. Check the temperature with a fat thermometer or place a cube of bread in the oil, it should rise to the surface immediately.
Gently slide about ten fish balls into the hot oil and fry for about 3 minutes, until evenly golden brown all around. Remove with a slotted spoon and drain on paper towels. Process the remaining balls in the same way.
For the dip, mix all the ingredients together. Arrange the fish balls in a serving bowl, garnish with the cilantro sprigs and cucumber slices, and serve piping hot. Serve the dip in a smaller bowl.