Cook the rice according to package instructions and set aside.
Make the sauce by mixing together the mirin, water, chicken powder, soy sauce, and sugar in a bowl.
Pour the sauce into a pan over medium heat. Add the sliced onions and let them soften for 2–3 minutes.
Add the chicken pieces to the pan. Let them cook in the sauce, flipping occasionally, until fully cooked and tender.
:Lightly whisk the eggs. Slowly pour them over the chicken and onions.
Cover the pan and let the eggs gently set to your desired doneness (slightly soft is best).
Spoon the chicken and egg mixture over a bowl of rice and garnish with chopped green onions.