Wash the cucumber. Clean and wash radishes. Cucumber and 4 radishes in wafer-thin slices. Line plates with them. For the vinaigrette, cut the remaining radishes into thin sticks.
Peel ginger and onion and dice very finely. Season vinegar with salt, pepper and sugar. Add prepared ingredients. Whip oil into it. Spread the vinaigrette on the plates. Cover and leave to stand for about 20 minutes.