Heat about 1cm of oil in a large heavy frypan over medium heat. While that is warming place the red onion onto a platter and sprinkle with balsamic vinegar.
Combine the breadcrumbs and flour. Press fish into the mixture to coat on both sides. Fry in the hot oil until cooked through, about 5 minutes on each side.
Transfer to the dish of onions and allow to drain. Wait 5 minutes and then transfer to serving plates. Spoon some of the vinegar and onions over the top for show.