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.
Top-ranked recipe named "Grilled Tuna Steak Teriyaki"
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).