Rinse the sushi rice under cold water until the water runs clear, which removes excess starch.
Cook the rice with 2 ½ cups of water in a rice cooker, or on the stove according to package instructions until fluffy. Let it cool down slightly.
In a medium bowl, mix shredded chicken with soy sauce, sesame oil, mirin (if using), and salt. Set aside to marinate for about 10 minutes.
Once the rice is slightly warm, dampen your hands with water (to prevent sticking) and take a handful of rice (about 1/3 cup).
Use your fingers to make a small indent in the center, then add a spoonful of the marinated chicken mixture into the indent.
Gently mold the rice around the filling into a triangular shape by pressing the rice together firmly but gently. Repeat this process until all rice and filling are used.
Sprinkle furikake over the onigiri for added flavor, and then wrap a strip of nori around the bottom of each onigiri for easy handling.
Finish with a sprinkle of chopped green onion for garnish.
Notes
Arrange the onigiri on a colorful plate and serve with pickled ginger or soy sauce on the side.