Cut the chicken breast into bite-sized pieces.
Slice the onion and green onion and set aside.
Add cooking oil to a pan and stir-fry the chicken breast.
When the chicken breast is about halfway cooked, add the onion and green onion and continue stir-frying.
Add soy sauce, sugar, minced garlic, and pepper, and mix well.
Add a little water and simmer until the sauce is reduced.
Finish by drizzling sesame oil if desired.