Rinse 2 cups of white rice under cold water until water runs clear. Cook rice in rice cooker or pot according to package directions until tender and sticky.

While rice cooks, prepare fillings. For salmon filling, season 200g salmon with salt and grill until cooked through, then flake into small pieces. For umeboshi filling, pit and chop pickled plum.

In a small bowl, mix 2 tablespoons rice vinegar with 1 teaspoon salt until dissolved. Once rice is cooked, transfer to a large bowl and gently fold in the vinegar mixture while rice is still hot. Let cool until warm enough to handle.

Prepare a small bowl of water with a pinch of salt for moistening hands. Cut nori sheets into strips about 2 inches wide and 4 inches long.

Wet your hands with salted water to prevent sticking. Take about 1/2 cup of warm rice and place in your palm. Make an indentation in the center and add 1-2 tablespoons of filling. Cover filling with more rice and begin shaping into a triangle or ball, applying gentle but firm pressure.

Continue shaping, rotating the rice in your hands, compacting it firmly so it holds together but is not overly compressed. The rice should be dense enough to hold its shape but still tender.

Wrap the bottom third of each onigiri with a strip of nori, leaving the top exposed. The nori should be crisp when served.

Repeat process with remaining rice and fillings. Serve immediately while rice is still slightly warm, or wrap individually in plastic wrap for later enjoyment.