Cook the rice according to the package. Mix together rice vinegar, sugar and salt in a saucepan and heat until sugar have dissolved. Let it cool off and then stir the mixture carefully into the cooked rice.
Remove the marinated tuna steaks from the refrigerator and rinse. Rub each tuna steak with the spice mix (you can also add little bit of oil to the mix). Either cook the steaks in a pan or on the grill for a few minutes on each side.