If you are not using Mentsuyu, combine and boil soy sauce, Sake, and Mirinin a small pan, remove from heat immediately, and let cool.

If the tuna is in a block, cut tuna sashimi into 1/2"(1cm) thick pieces.

Marinade Tuna pieces in cooled soy sauce mixture (or Mentsuyu) for 10-15 minutes.

Serve rice in a bowl, and top with marinaded tuna, sesame seeds, wasabi, and Roasted Seaweed. Pour some of the marinade sauce over rice if you want.