The Treviso type of radicchio is another variety of this vegetable, the underground parts are also used. Peel 4 heads, chamfer the outer part of the stem, cut them into small pieces and pour them into a bowl. Peel an apple and cut it into quarters. The one I use is an old variety, with a bitterish skin, but sour apples like Smith or Annurche are also good. Apples tend to oxidize easily, so, as soon as they are peeled, acidify them by rubbing them with a lemon divided in half. We cut the apple into small cubes and add it to the red lettuce, after which we take the Parmigiano and flake it on the top, coarsely. I recommend that the Parmesan is at least 30 months, a little mature, if not the flakes will crumble. A little oil, a little salt, and mix it up.