Thinly slice the onion and put in a small bowl of cold water with the vinegar (this helps to get rid of the acidity in the onion). Leave to soak for 20 minutes or so. Cut the apple into 2-3 mm (1/16-⅛ inch) slices and put in a bowl of cold water with the lemon juice and a sprinkling of salt.
Cut away the thick white spine from the radicchio, then tear up or shred the leaves thickly. Div