Transfer hot cooked short-grain rice into a large bowl and season with salt with a rice paddle. Cover and set aside. (If you're using leftover cold rice, cover and microwave it for 2-3 minutes prior to seasoning).

Open a can of tuna and strain the tuna using a fine sieve removing as much of the liquids as possible by pressing it with a fork. Transfer the strained tuna to a small bowl. Mix in the Japanese mayo until combined. Set aside.

For the nori sheets, the amount used is personal preference. If you prefer more dry roasted seaweed, use two nori sheets and cut them into half so you have 4 pieces. If you prefer less nori, use half a sheet and cut them into 4 equal strips. Set aside.

Prepare a bowl of water nearby and dip clean hands into the water. Take ½ cup or 100 grams cooked rice and place it into the palm of your hand. Shape it into a round ball, then flatten into a ½-inch thick pancake. Make a dent in the middle. Place 2 tbsp or 30 ml of tuna mayo mixture into the dent. (*If you have an onigiri mold or a large sheet of plastic wrap, use these tools to help easily mold the rice balls to prevent a sticky mess)

Fold the rice over the filling and as you fold, push the filling into the center. Firmly pack it into a ball, flatten it so it's 1.5-inch thick and shape it into a triangle, wetting your hands as needed.

Wrap a sheet nori from the base of the triangle around the sides and repeat this process until you have 4 rice balls in total. Garnish each piece with sesame seeds. Enjoy immediately!