Let's cook.
In a medium bowl, combine the drained tuna, mayonnaise, soy sauce, and finely diced cucumber. Mix well until thoroughly combined.
Lightly wet your hands with water to prevent the rice from sticking. Sprinkle a pinch of salt onto your palms.
Take about 1/2 cup of the cooled cooked rice and flatten it into an oval shape in one hand.
Place about 1 tablespoon of the tuna mixture in the center of the rice.
Gently bring the edges of the rice together to cover the filling, then carefully shape the rice into a triangle or oval, pressing firmly but not too tightly.
Repeat with the remaining rice and tuna mixture to make 4 onigiri.
Wrap each onigiri with a strip of nori, pressing it gently to adhere to the rice. Serve immediately or pack for later.
Save this recipe so you can find it again.