Combine first 3 ingredients in a heavy-duty zip-top plastic bag; add tuna. Seal and chill 1 hour, turning occasionally.
Remove tuna from plastic bag, discard marinade.
Grill tuna, covered with grill lid, over high heat (400° to 500°) 2 minutes on each side or to desired degree of doneness. Serve with steamed asparagus. Garnish, if desired.
I really wasted a beautiful piece of Ahi. Way too soy saucey flavored. Should have stuck with my original plan..the sesame coated tuna recipe from Cooks Illustrated