Ready in 45 minutes
Fast and easy. I did this on the indoor grill and it turned out really good, including those nice grill marks. The marinade recipe is an adaptation from Steve's Chicken Teriyaki ala Steve.
Combine all ingredients except the tuna in a ziploc bag.
Add the tuna and turn the bag over a few times to make sure it's well coated with the marinade.
Marinate about one hour.
Grill to your preferred doneness. I don't like my tuna rare, so I gave it about five minutes per side, which made medium (but still pink in the center).