Combine chicken, soy sauce, mirin, and sugar in a bowl and set aside for 5-10 minutes while preparing the other ingredients.
To prepare the egg mixture, crack the eggs into a bowl and gently break the yolks using a pair of chopsticks. You want to make sure the eggs are lightly beaten with the egg whites still visible.
In a small pan, bring chicken stock to a boil over medium heat. Add onion and cook until aromatic, about 1 minute.
Add the chicken mixture and cook for about 1-2 minutes on each side.
When the chicken is almost cooked through, pour the egg mixture evenly into the pan and cook for a few seconds until the edges are nearly set.
Lightly scramble the eggs and cook for a few seconds more until the eggs are still a bit runny - the residual heat will continue to cook the eggs.
Serve over a bowl of rice, garnish and enjoy!
