Dry the tuna steaks with paper towels. Salt them well on both sides, the dredge them in flour and shake off the excess.
Heat the oil over medium heat in an 8- to 9- inch skillet. When the oil is so hot that it is rippling, sear the tuna steaks for 2 minutes on each side, and briefly on the edges. Transfer to a deep serving dish just large enough to hold the tuna and, eventually, the onions.
As soon as the tuna is out of the pan, add the onions and 3 tabespoons of water. Cook the onions for about 3 minutes, until the water evaporates. Sprinkle on the sugar and salt. Toss constantly until the onions begin to caramlize, about another 5 minutes.
Add the wine and scrape up any caramalized specks. Add the vinegar and simmer for 1 minute. Pour the onions and their juices over the tuna. Let stand until ready to serve, at least 1 hour. (or let stand for an hour, then refrigerate the tuna - for up to 24 hours - until 15 minutes before you are ready to serve it.)
Served garneished with oil-cured olives and chopped fresh parsley.