Make two triangle-shaped onigiri rice balls. Lightly spread a small amount of oil in a non-stick frying pan and heat over medium heat. Cook the onigiri for 2–3 minutes without touching, until the surface is lightly browned. Turn over, brush soy sauce on the rice, and cook for another 2–3 minutes. Flip again, brush soy sauce on the other side, and cook for 2–3 minutes more. Repeat this process a couple more times until the rice is nicely browned and crispy.
Meanwhile, heat the dashi in a saucepan over medium heat and season with sake, soy sauce, and salt. Keep it warm until ready to serve.
For the toppings, remove the pit from the umeboshi plum and chop it into a paste. Thinly slice the green onion and cut the nori into very thin strips.
To serve, place the grilled onigiri in a bowl, pour the hot broth over, and add the toppings. Serve immediately.