Combine the brown sugar, soy sauce, garlic, ginger, and mirin together in a bowl.
Heat the oil in a large non-stick skillet. Add the chicken to the skillet and stir fry until seared, around 5 minutes. Remove the chicken from the pan and set aside.
Add the soy mixture to the pan and bring it to a boil over high heat. Keep stirring the sauce until it begins to thicken and become sticky, around 5 to 6 minutes.
Add the chicken back in to the pan, and add the green onions. Stir everything together until the sauce coats the chicken. Keep cooking until the sauce is reduced.