Squeeze two oranges and collect the juice. Peel and thinly slice the onion, transfer it to a pan with the sugar and the clove. Add the orange juice, turn on a very low flame and cook for 30 minutes with the lid on. Turn off the heat, remove the clove and leave to cool down completely. Cut the remaining grapefruits and oranges, slice them and alternate them on individual plates. Add the aged strong Provolone Valpadana P.D.O. cut into slices with a potato peeler, then sprinkle with chopped pistachios. Complete with a drizzle of oil, a few strands of chives and serve.