Cut the chicken thigh into bite-sized pieces.
Slice the onion thinly.
Beat the eggs and set aside.
Heat oil in a pan and stir-fry the chicken thigh.
When the chicken is halfway cooked, add the onion and stir-fry together.
Add soy sauce, mirin, sugar, and water. Bring to a simmer.
When the onion and chicken are cooked through, pour in the beaten eggs.
Cover the pan and cook gently over low heat until the eggs are set.
Serve the cooked mixture over rice.