Divide the pomegranate in half, tap on the outer skin of one half with a wooden spoon so that the grains fall out (work over a bowl or a plate, so that the grains fall onto it). Then put them in a strainer to drain. Wash the three types of radicchio, cut them into thin strips and put them in a large bowl together. Add the grapes, cut in half (if you don’t like the seeds remove them with the tip of a knife), the grains of pomegranate and the walnuts. Beat the oil, salt and lemon juice together in a bowl; pour over the salad and mix well. Grate the pecorino in flakes and add it to the salad, leaving a handful to one side. Mix carefully and tip the salad onto a serving dish. Before serving, garnish with the remaining flakes of cheese.