If necessary, cut the hot pot beef slices into bite-sized pieces. Set aside.
In a bowl or measuring cup, combine the Japanese soy sauce, mirin, sake, dashi granules and sugar. Stir well.
Add the sauce and the ginger slices to the beef and stir to mix. Let sit for 15 minutes.
While waiting for the beef to marinade, add the rice to a mesh colander and rinse under cool water for about a minute, stirring the rice around with your hand, until the water is mostly clear. Set aside to drain.
Transfer the marinated beef, ginger slices, and the sauce to the rice cooker pot.
Layer the onions on top of the beef.
Add the rice to the pot even it out. Add the water.
Close the rice cooker lid and set it to Cook.
After the rice cooker has switched off, allow the rice to sit uncovered for another 10 minutes.
Stir the beef and rice together. Remove the ginger slices.
Serve hot. If desired, garnish with pickled ginger, Shichimi Togarashi red pepper mix, chopped green onions, etc.
