Rinse the Japanese Short-Grain Rice under cold running water, swishing until the water runs clear; drain, then combine with Water in a rice cooker or heavy pot (use the ratio recommended for your rice). Let the rice soak 20–30 minutes if you have time, then cook according to your rice cooker or simmer covered on low until tender; rest, covered, 10 minutes off the heat.
Fluff the cooked rice and season gently with a small pinch of Fine Sea Salt to taste; keep the rice warm and covered while you make the fillings.
Prepare fillings: for salmon, pat the Salmon Fillet dry, sprinkle lightly with salt and pan-sear or broil until just cooked through, then flake; toss the flakes with a few drops of Soy Sauce and a sprinkle of Katsuobushi for extra umami. Alternatively, use a pitted Umeboshi whole for a bright, tangy center, or spoon in a little Kombu Tsukudani for a rich, savory filling.
To form onigiri, have a small bowl of Cold Water and a pinch of salt at hand. Wet your hands to prevent sticking, sprinkle your palm with a little salt, then take a handful (about 1/2 cup) of warm rice and make a slight indentation in the center.
Place about 1 teaspoon (or more, to taste) of your chosen filling into the indentation, cover with another small scoop of rice, and shape the ball into a tight triangle by cupping your hands and pressing gently but firmly. Avoid compressing so hard that the rice turns paste-like.
Wrap the base or one side of each onigiri with a strip of Nori Sheets just before serving to keep it crisp. Optionally, finish with an extra sprinkle of katsuobushi or a tiny pinch of salt.
Serve the onigiri immediately, or wrap individually in plastic wrap and store in the fridge for up to a day (bring back to room temperature before eating for best texture). Enjoy warm or at room temperature as a portable, flavorful snack.
