1.Combine tuna, soy, wasabi, juice and sugar in medium bowl. Drain tuna; reserve 2 tablespoons of the marinade.
2.Place noodles in large heatproof bowl, cover with boiling water; separate with fork, drain.
3.Heat half the oil in wok; stir-fry tuna, in batches, until browned and cooked as desired. Remove from wok.
4.Heat remaining oil in wok; stir-fry cabbage until wilted. Add noodles, reserved marinade, sweet chilli sauce and fish sauce; stir-fry until mixture boils, season to taste. Serve noodles topped with tuna and onions.
Salmon or ocean trout can be used in place of the tuna.