Peel the onions, cut one into thin rings and finely dice the other. Peel the oranges as well, removing the furry inner skin. Use a sharp knife to remove the fruit fillets from the separating membranes. Squeeze the juice out of the separating membranes, mix with balsamic vinegar, salt and cayenne pepper, only then whip in both types of oil. Halve the radicchio heads, wash, pat dry thoroughly and cut across into 2 cm wide strips. Mix with the salad dressing and the diced onion. Arrange the onion rings and orange fillets decoratively on four plates, place the salad in the middle and sprinkle with shaved hazelnuts.