1. Preparation
Rinse 300g of short-grain rice under cold water until the water runs clear to remove excess starch.
Soak the rinsed rice in water for 30 minutes while preparing other ingredients.
Dice any desired fillings such as 100g of umeboshi (pickled plum) or 100g of cooked salmon into small pieces.
Prepare a bowl of water mixed with a pinch of salt for handling the rice and shaping the onigiri.
2. Cooking
Drain the soaked rice and transfer it to a rice cooker or pot.
Add 360ml of water to the rice cooker or pot.
Cook the rice according to the rice cooker's instructions, or if using a pot, bring it to a boil, then reduce heat, cover, and simmer for about 15 minutes until the water is absorbed.
Once cooked, let the rice sit covered for an additional 10 minutes to steam.
While the rice is still warm, transfer it to a large bowl and use a wooden spatula or paddle to fluff it gently and cool to a manageable temperature.
3. Shaping
Moisten hands with the salted water to prevent sticking.
Take about a handful of rice (approximately 100g) and shape it into a triangle or oval, creating a small indentation in the center.
Place a spoonful of filling in the indentation and cover it with rice, then reshape the onigiri so that the filling is completely enclosed.
Wrap the formed onigiri with a piece of nori, if desired, and repeat the process for remaining rice and fillings.