Combine soy sauce, mirin, sesame oil, tobanjan, and sugar in a medium mixing bowl and stir until sugar dissolves.
Add diced tuna and avocado to sauce in bowl, toss well, and let marinate for up to five minutes (but no more than that!).
Divide cooked rice between wide bowls and top with tuna and avocado mixture, leaving sauce behind.
Drizzle sauce over the tuna/avocado.
Tear nori sheets into small pieces and sprinkle over the bowls. Top with shiso leaves and sesame seeds.
Enjoy immediately!