Place a sieve over a bowl, add the rice, and rinse it with water. Repeat this process several times, changing the water each time.
Soak the rinsed rice in water in a pot for at least 30 minutes.
Put water (as specified in the recipe) in a pot and bring it to a boil. Once it comes to a boil, reduce the heat to low, add the bonito flakes, and let it simmer for 2 minutes.
Turn off the heat and strain the mixture through a sieve lined with paper towels or cloth (such as cheesecloth). Bonito dashi is ready.
Cut the bamboo shoots into bite-sized pieces along the fibers for the upper part, about 0.2 inch (5mm) thick. For the bottom to middle part, slice them into bite-sized pieces against the direction of the fibers, about 0.2 inch (5mm) thick. Finely chop the aburaage.
Discard the water in which the rice was soaked from the pot. Combine the drained rice with the dashi (⅔ cup (160ml) per 2 servings) and seasonings (sake, mirin, salt, soy sauce). Then, evenly distribute the bamboo shoots and aburaage on top.
Cover the pot and bring it to a boil. Once it comes to a boil, reduce the heat to low and continue cooking for 10 minutes, or slightly longer for larger quantities (11 minutes for 4 servings, 12 minutes for 6 servings).
Turn off the heat and let it steam for 10 minutes with the lid still on.
Serve in each bowl and top with the sansho leaves for color (optional).