Add the sake and hon-mirin to a small pot and bring to a boil to burn off the alcohol. Allow to cool.
Cut the tuna into bite-sized pieces. Remove the stem of the shiso leaves and mince.
Once the ingredients from Step 1 have cooled, mix in the soy sauce, then add the tuna from Step 2. Wrap in plastic wrap and let marinate in the refrigerator for 1 hour.
Turn over once to allow both sides to marinate. Add some rice to a bowl, sprinkle in the nori, then add the tuna and shiso. Top with black pepper and it's complete.
Please serve with wasabi.
Story Behind this Recipe
I had a similar dish in Tokyo that was really delicious, so I asked the chef about it. His response was that the flavor of the tuna changes completely depending on the tuna used, so I couldn't recreate the flavor. Well, that's what he said, but this is good enough for me!
Don't marinate for too long, or else the soy sauce will make it too salty. This is also delicious with other fish, such as amberjack and bonito!