Combine the red wine, shallot, and vinegar in a nonreactive heavy saucepan and simmer over low heat until reduced to a syrupy consistency.
Slowly incorporate the cold butter into the sauce while simmering, stirring with a wooden spoon and adding one piece of butter at a time; add the next piece when the previous one has just melted into the sauce.
Remove the shallot and season the sauce. Keep warm until needed.
Tuna lends itself very well to some of the same methods of cooking as beef. This preparation could confuse a blindfolded cowboy into thinking he was biting into a succulent steak.
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