re: Cooking a Tuna Steak...

Can't go wrong with salt, pepper and a little olive oil. Maybe 60 seconds at most on each side depending on how thick the steak is.

I've tried this before and it turned out tasting great.
½ teaspoon of Japanese horseradish, wasabi powder,
¼ cup of soy sauce, and
½ cup of honey
Just combine the 3 and marinate the steaks for about 30 minutes then sear them on the grill for about 1 minute. The wasabi adds a great element to it.