In a small pot, combine the soy sauce, Sake, and Mirin and bring to a boil. Remove from heat immediately, and let cool.
Cut the 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 marinated tuna, sesame seeds, wasabi, and Roasted Seaweed. Pour some of the marinade sauce over rice if you want.