One of the most desirable elements of an antipasto spread is sweet and sour onions. For this the best is the borettane onion, a small flat onion the size of a plum, which is grown all over Italy. Using balsamic vinegar to pickle these onions makes them irresistible, but beware, they are not pickled enough to enable you to keep them for a long time.
Heat the olive oil in a large saucepan and fry the onions gently until soft, about 10 minutes. Add the vinegar, sugar, salt and a few grates of nutmeg, and continue to fry very gently for 15 minutes.
Eat straight away, or put into a clean jar and keep in the refrigerator for a few days only.