Add the rice and water to a rice cooker or pot and cook according to standard instructions. This will yield enough for 9 onigiri.
In a cold skillet, place the frozen gyoza and add just enough water to cover the bottom. Drizzle with oil, cover, and cook over medium heat until the gyoza are cooked through and the bottoms are golden and crispy. Add more water as needed during cooking.
Transfer the cooked rice to a large bowl and let cool slightly until warm but comfortable to handle.
Wet your hands (or an onigiri mold) with water to prevent sticking. Sprinkle lightly with salt.
Scoop a handful of warm rice into your palm or mold. Place one gyoza in the center, then cover with more rice. (Optional: dip the gyoza in sauce before sealing inside.)
Gently press and shape into a triangle using your hand or mold.
