I was going to tell you to be sure and freeze it first so good going there!!!!
I would also be tempted to cut them up. You don't want the thinner parts to be over-cooked and dry.
Kabobs come to mind - marinate chunks in olive oil, lemon, garlic, rosemary (not too much), or something like a ponzu sauce i.e., soy sauce, orange juice, olive oil, lemon, parsley or cilantro...there's so many ways to marinate - - soy sauce, brown sugar, rice vinegar, sherry, ginger, garlic, spring onions, sesame oil, sesame seeds...of course, any of these marinades will work for tuna steaks too - or even a whole tuna if you still want to go that route.
When doing kabobs I always cook the veggies separately - all veggies on the same stick. I like my tuna very rare but I want my veggies caramelized.
"Count yourself...you ain't so many" - quote from Buck's Daddy