Set a colander with holes smaller than the rice grains or a sieve in a bowl that holds it snugly. Mix the rice grains in the colander. Add enough cold running tap water to cover by an inch, swirl the rice using your fingers and lift out the colander as quickly as possible. Pour out the water. Repeat until the water is clear after swishing, 4 to 7 times.
Shake the rice in the colander to remove excess water, then pour into a small donabe rice pot or medium saucepan. Add the cold filtered water, stir once with your fingers and let soak for 30 minutes.
Cover the pot and place over high heat. Bring to a rolling boil, 10 to 12 minutes. The moment the water starts to boil, reduce the heat to medium-low and cook the rice at a steady simmer for an additional two minutes. Remove the pot from the heat and let stand, covered, for 20 minutes.
Uncover the pot and let stand until the rice is cool enough for you to work with your hands. The hotter the rice, the easier it is to mold, but don’t burn yourself. Use a rice paddle or flat, wide spatula to gently mix the rice without crushing the grains. You just want to even out the grains and disperse the moisture from the center of the pot.
Spoon a paddleful of rice (about 1/3 cup) into a small rice bowl. Drop a teaspoon of filling in the center, then top with just enough rice to cover the filling.
To shape onigiri by hand: Rinse your hands with cold water and shake off excess. Sprinkle a pinch of sea salt on the palm of one hand, leaving the salt on the fingertips of your other hand. Turn the rice out of the bowl into the palm with the salt. Use the heel of your other palm and your fingers with salt to gently squeeze and press the rice into a 1-inch-thick triangle shape, rotating the rice as you press it.
Alternatively, shape onigiri in plastic wrap: Place a large sheet of plastic wrap on your work surface. Sprinkle a pinch of salt evenly over the rice in the bowl, then turn the rice out of the bowl into the center of the plastic wrap. Bring up the sides around the rice and twist and wrap tightly. Gently squeeze the rice in the plastic wrap to form a 1-inch-thick triangle shape.
Turn on a gas stovetop burner to medium heat. Pinch a sheet of nori on one end with your fingers and wave and flap it 3 to 4 inches over the flame until fragrant and slightly crisper, 15 to 20 seconds. Immediately wrap it around the rice and eat as soon as possible.