Peel onion and cut into half rings. Clean and wash the bell peppers and cut them into thin strips. Cut cheese into small sticks. Clean, wash and quarter the radicchio, also cut into fine strips.
For the sauce, mix mustard, vinegar and oil. Season with salt, pepper and sugar. Carefully mix onion, paprika, cheese and radicchio salad with the sauce. Wash and sort the lamb's lettuce. Clean and wash the oak leaf and pluck into bite-sized pieces.
Arrange plates with salads. Spread cheese salad on top.