1.Cut tuna into 2cm (¾-inch) cubes or slice thinly. Place tuna in medium bowl with vinegar, stand 15 minutes.
2.Meanwhile, make miso dressing. Stir miso, mirin, sake and sugar in small saucepan over heat, without boiling, until sugar dissolves. Remove from heat, stand 10 minutes. Stir in vinegar, sauce and mustard. Cover, refrigerate until required.
3.Drain tuna, discard marinade. Pat tuna dry with absorbent paper. Place tuna in medium bowl with chilled dressing, toss gently to coat.
4.Divide tuna mixture among serving bowls, sprinkle with onion. Serve with wasabi or japanese mustard, if you like.
Sashimi salmon may be used instead of tuna.Note