Scrape the skin off the fish and discard. Bone the fish carefully and cut it into 2-inch (5-cm) pieces. Rinse and dry. Thinly slice the onions. Put the oil in a pan that will later hold all the fish pieces in a single layer. Add the onions and sweat them over medium heat for 6 to 7 minutes.
Put the fish in the pan with the onions and cook over low heat for about 6 minutes, turning gentl