Wash the potatoes and cook in water for about 20 minutes. Quench cold, peel and let cool. In the meantime clean and wash the radicchio and cut into bite-sized pieces. Cut cucumbers into small cubes.
Mix mustard, vinegar and cucumber water. Season with salt, pepper and sugar. Fold in the oil. Wash oregano, dab dry, pluck leaves from the stalks and chop finely. Add oregano, cucumber cubes and capers to the vinaigrette and stir in.
Cut the potatoes into slices and mix with radicchio, mortadella and the vinaigrette. Allow to soak a little and season again. Arrange on a platter and serve garnished with oregano as desired.