Season Rice:

Place hot cooked short-grain rice in a large bowl and mix in salt with a rice paddle. Cover and keep aside. (If using cold leftover rice, cover and heat in microwave for 2-3 minutes before seasoning.)

Prepare Tuna Mayo:

Open the tuna can and drain the contents in a fine sieve, pressing with a fork to extract as much liquid as possible. Put the drained tuna into a small bowl and stir in Japanese mayo until evenly coated. Set aside.

Cut Nori Sheets:

Adjust nori sheets to taste: for more seaweed, use 2 sheets and cut each into two halves for 4 pieces; for less, use 1/2 sheet and cut into 4 equal strips. Set aside.

Form Rice Patty:

Have a bowl of water ready. Wet your hands, then take 1/2 cup (100 g) cooked rice and form it into a round ball. Flatten to a 1/2-inch thick patty and create an indentation in the center. Add 2 tbsp (30 ml) of the tuna-mayo mixture into the dent. (Using an onigiri mold or plastic wrap can help avoid stickiness.)

Shape Onigiri:

Fold the rice over the filling, pushing the filling toward the center as you fold. Press firmly into a ball, then flatten to 1.5-inch thick and shape into a triangle, rewetting hands as necessary.

Wrap and Serve:

Place a nori sheet starting from the base of the triangle and wrap around the sides. Repeat to form 4 onigiri. Sprinkle each with sesame seeds. Serve immediately.