1. Preparing the Rice
Measure 1 cup of Japanese short-grain rice and rinse under cold water until the water runs clear.
Drain the rice and let it sit for about 30 minutes to allow it to absorb some water.
Add the rinsed rice and 1 1/4 cups of water into a rice cooker. Let it soak for about 10 minutes before cooking.
Cook the rice according to the rice cooker's instructions or bring to a boil, then reduce to low heat, cover, and simmer for 15 minutes.
2. Preparing Umeboshi Filling
Take 2-3 umeboshi and remove the pit.
Mash the umeboshi with a fork or chop finely until it reaches a paste-like consistency.
3. Shaping the Onigiri
Once the rice is cooked, let it rest for about 10 minutes with the lid on.
Moisten your hands with water to prevent sticking and take a handful of rice (about 1/4 cup).
Make a small indentation in the center of the rice and place about 1 teaspoon of umeboshi filling in the hole.
Carefully shape the rice around the umeboshi, pressing gently to form a triangle or ball shape.
Repeat for the remaining rice and filling.
4. Serving
Optionally, wrap each onigiri with a strip of nori (seaweed) for added flavor and presentation.
Serve the onigiri warm or at room temperature, garnished with sesame seeds or alongside pickled vegetables if desired.