First marinate tuna for at least an hour in:

1/4 cup of soy sauce
1/4 cup of red wine
1/4 cup of red wine vinegar
3 TBS sesame oil
couple of cloves of garlic cut up

Sauce (measures are approximate):

bunch of ginger cut up finely (I use ginger powder - at least a
      teaspoon)
2-3 scallions cut up finely 
1/2 cup rice vinegar
1/2 cup white wine
3/4 pound of butter (I probably use about 1/2 of this, maybe less)
1 cup heavy cream (I use half and half, probably close to this much)

In a pan, cook the first 4 ingredients until they cook down to one or
two tablespoons (stir occasionally).  Then melt the butter into it
followed by the half and half and cook while stirring occasionally.
That's it.  About 5 to 10 minutes to prepare the sauce.

Cook the tuna.  Fan the tuna out and serve the sauce over it in a
restaurant-style.  

Enjoy!