Cook the rice per the instructions on your rice package.
While the rice is cooking, drain the can of tuna and add to a small mixing bowl along with the Japanese mayo. Mix together with a fork until combined.
Once the rice is done cooking, cut the nori sheet into three evenly sized strips. Then cut two strips in half and place aside until needed. Be sure to use high quality, fresh nori so it will be nice and crispy.
When you're ready to assemble the onigiri, wet your hands with warm water and rub a little bit of salt on them prior to handling the rice. This helps to season the rice and keep it from sticking to your hands.
Then take a handful of rice into your hand and flatten it slightly, making a small well in the middle for the filling.
Add a spoonful of the tuna and mayo mixture to the well in the middle of the rice and cover with another small handful of rice.
Then form the tuna rice ball into a triangle shape by cupping your hands and gently pressing the rice with your palm and fingers (see video for more detail). Most beginners tend to press too hard when shaping the rice. Instead, you want to gently use your hands to form the rice into a triangle shape.
Next, wrap a half strip of the nori around the bottom center of the onigiri.
Garnish with black sesame seeds, serve, and enjoy!