1.Place onion and chilli in a bowl of iced water to curl.
2.Meanwhile, for cucumber lime dressing, combine cucumber, lemongrass, chilli, shallots, juice, sugar and fish sauce in a medium bowl; stir until sugar is dissolved. Stir in peanuts and coriander.
3.Toss fish in seasoned flour; shake away excess. Shallow-fry fish, in batches, in hot oil, until just cooked through. Drain on absorbent paper.
4.Serve fish topped with cucumber lime dressing, drained onion and chilli and coriander.