Wash rice in a mesh strainer until water runs clear. Combine washed rice and 4 1/2 cups water in a saucepan. Bring to a boil over high heat, stirring occasionally. Reduce heat to low; cover, and simmer rice until water is absorbed, 15 to 20 minutes. Let rice rest for 15 minutes to continue to steam and become tender. Allow cooked rice to cool.

Combine the remaining 1 cup water with salt in a small bowl; use to dampen hands before handling rice. Divide cooked rice into 8 equal portions. Use one portion of rice for each onigiri.

Divide one portion of rice into two. Create a dimple in rice and fill it with a heaping teaspoon of bonito flakes. Cover with the remaining portion of rice and press lightly to enclose the filling inside the rice ball. Gently press the rice to shape it into a triangle; wrap it with a strip of nori and sprinkle with sesame seeds. Repeat with remaining portions of rice.