Cut the chicken thighs into bite-sized pieces.
Julienne the onion.
Lightly beat the eggs in a bowl.
Heat oil in a pan and cook the chicken thighs.
When the chicken is half cooked, add the onions and stir-fry.
Add soy sauce, mirin, sugar, and water, then simmer.
Pour in the eggs, cover with a lid, and cook over low heat.
Serve the cooked chicken and egg over rice to finish.