Drain 70g of canned albacore tuna and flake it. Mix with 1 tablespoon of Japanese mayonnaise and ¼ teaspoon of soy sauce.
Divide 2¼ cups of warm cooked Japanese short-grain rice into ¾-cup portions. Line a small bowl with plastic wrap and sprinkle with a little of the ½ teaspoon of kosher salt.
Add half a portion of rice, a spoonful of the tuna mayo, and the rest of the portion. Sprinkle with a little more salt.
Twist the wrap closed and press the rice firmly into a triangle, one corner at a time. Repeat with the remaining rice.
Cut 1 sheet of nori into thirds. Wrap a strip around the base of each onigiri, leaving the top corner bare, and top with the remaining tuna mayo.
