Make Zuke sauce. Combine all the ingredients in a small pot and let boil for a minute. Pour into a small container and cool.
Slice Sashimi grade tuna thinly, and marinate in the Zuke sauce for 20 minutes. Keep refrigerated until using it.
Cut Shiso leaves very finely.
Serve 1 cup steamed rice in a bowl. Top with Nori roasted seaweed on the rice. Place half of the marinated tuna on top. Spoon some of the Zuke sauce over. Sprinkle sesame seeds on the tuna. Put a small mound of cut Shiso leaves in the middle, and add Wasabi. Assemble another one.
