Clean and wash the radishes and cut them into thin strips. Clean and wash spring onions and cut them diagonally into thin rings. Cut salmon into fine strips. Clean and wash the cucumber and cut it in half lengthwise.
Remove the cucumber seeds with a teaspoon. Cut 1 half of the cucumber into large pieces. Finely puree the cucumber pieces, yoghurt and oil with the cutting stick of the hand mixer or in a blender. Cut the remaining cucumber half lengthwise into narrow strips and dice finely.
Add cucumber cubes, onion rings, radish and salmon strips to the yoghurt soup and stir into the yoghurt soup. Season soup with salt and pepper to taste. Divide the yoghurt soup into four deep plates. Garnish with a few dill flags if desired.