Cut the chicken and onion into bite-sized pieces.
Crack the eggs into a bowl and beat them well.
Place soy sauce, mirin, sugar, and water into a pan and bring to a boil.
Add the onions and chicken and cook over medium heat.
Once the chicken is cooked, pour in the beaten eggs and cook until the eggs are softly set.
Place the cooked ingredients over rice and garnish with chopped green onion.