Peel the onion and cut it into rings. Add wine (grape) vinegar and water to the sugar and salt. Salt is better to take sea salt. If the usual, then a little less, since the sea is less salty. Heat, stir and prepare the marinade. Put the chopped onion in a bowl, if desired, add a sprig of rosemary and pour over the hot marinade. Leave to marinate for a day.