Cut the chicken breast into bite-size pieces.
Slice the onion thinly.
Beat the eggs and prepare them.
Heat oil in a pan and stir-fry the chicken breast and onion.
Add soy sauce, mirin, sugar, and water, and simmer until the chicken breast is cooked.
Pour the beaten eggs over, cover with a lid, and cook gently over low heat.
Serve over rice to finish.