Place the 2 cups japanese short-grain white rice (如越光米或加州米) in a Sieve or Mixing Bowl and rinse under cold running water, gently swishing with your hand, until the water runs nearly clear — about 3 to 4 rinses. This removes excess surface starch and is the single most important step for great onigiri rice. Drain well.

Combine the rinsed 2 cups japanese short-grain white rice (如越光米或加州米) and 2¼ cups of 2 ¼ cups water in a Saucepan. Let the rice soak for 10 minutes before cooking — this helps it cook evenly and become perfectly tender. (If using a Rice Cooker, add rice and water and cook on the white rice setting; skip steps 3–4.)

Bring the 2 cups japanese short-grain white rice (如越光米或加州米) and 2 ¼ cups water to a boil over medium-high heat with the lid on. As soon as it reaches a full boil, reduce the heat to the lowest setting and cook, covered, for 13 minutes. Do not lift the lid during cooking.

Remove from heat and let the 2 cups japanese short-grain white rice (如越光米或加州米) steam, still covered, for 10 minutes. This resting step is essential — it finishes the cooking and keeps the rice fluffy. Then uncover, add 1 tsp 1 tsp salt (用于调味米饭), and gently fold it in with a Spatula using a cutting motion (don't stir or mash). Spread the rice out slightly to let steam escape, then let cool until just warm enough to handle, about 10–15 minutes.

While the rice cooks and rests, prepare the 1 can of tuna, 沥干 (水浸，充分沥干) mayo filling. Drain the canned 1 can of tuna, 沥干 (水浸，充分沥干) very well — press it in the Colander with the back of a Wooden Spoon to remove as much liquid as possible. In a Mixing Bowl, combine the drained 1 can of tuna, 沥干 (水浸，充分沥干), 2 tbsp japanese mayonnaise (如丘比), 1 tsp soy sauce, and a pinch of 1 tsp salt (用于调味米饭) if using. Mix well and taste — it should be creamy, savory, and slightly rich. Set aside.

If your 4 pieces of umeboshi, 腌制 (去核的日式腌梅；每个饭团1个) have pits, remove them now. Each 4 pieces of umeboshi, 腌制 (去核的日式腌梅；每个饭团1个) will be used whole as one filling per onigiri. Their tangy, salty, and slightly fruity flavor is a beloved classic — no additional prep needed.

Set up your shaping station: a Mixing Bowl of 2 ¼ cups water mixed with a generous pinch of 1 tsp salt (用于调味米饭) (for your hands), the warm 2 cups japanese short-grain white rice (如越光米或加州米), your fillings, and the Nori strips laid out on a clean surface. Wet both palms with the salted 2 ¼ cups water — your hands should be damp but not dripping. This prevents the rice from sticking and lightly seasons the outside of the onigiri.

Scoop about ½ cup (roughly 100–110g) of warm 2 cups japanese short-grain white rice (如越光米或加州米) into one palm and flatten it slightly into a disc. Make a small well in the center with your thumb. Add about 1 tbsp of 1 can of tuna, 沥干 (水浸，充分沥干) mayo filling (or 1 4 pieces of umeboshi, 腌制 (去核的日式腌梅；每个饭团1个)) into the well. Gently bring the edges of the rice up and around the filling to enclose it completely, forming a rough ball.

To shape into a triangle: cup the rice ball in both hands. Use the fingers and thumb of one hand to form a V-shape (like a peace sign), and use the palm of the other hand as a flat backing. Gently press and rotate the rice ball, applying even, firm pressure to form three flat sides and three defined corners. Aim for a compact, uniform triangle — rotate and press 4–5 times. Don't squeeze too hard; the rice should hold together but stay light, not dense. Re-wet hands between each onigiri.

Wrap the base of each onigiri triangle with a strip of Nori, shiny side out, pressing gently so it adheres. The nori should cover the bottom third of the triangle, giving you a handle to hold while eating. Sprinkle with toasted 1 tbsp sesame seed, 烘烤 (白色或黑色) if desired. Repeat with remaining 2 cups japanese short-grain white rice (如越光米或加州米) and fillings to make 8 onigiri total (4 tuna mayo, 4 umeboshi).

Serve immediately while the 2 cups japanese short-grain white rice (如越光米或加州米) is still slightly warm and the Nori is crisp. If packing for later, wrap each onigiri individually in Plastic Wrap to keep them moist. For the crispest nori, store the nori strips separately and wrap just before eating.