4 tbls Dijon-style mustard
2 6-oz tuna steaks
salt to taste
2 large onions, thinly sliced
pepper, freshly ground
2 tbls balsamic vinegar
to taste

Combine mustard, salt and pepper.
Rub the mixture on both sides of the tuna steaks.
Cook the tuna over high heat in a nonstick pan to seal in the juices.
(A 1″ thick tuna will take 3 to 5 minutes on each side.)
Remove the tuna.
Set aside.
Add the onions to the pan.
Sear for 3 to 4 minutes.
Deglaze the pan with the vinegar.
Place the tuna on dinner plates.
Spoon the onion-vinegar mixture over.