Add light soy sauce, toasted sesame oil, dark brown sugar, rice vinegar, red pepper flakes, sesame seeds, onions, and garlic to a bowl. Whisk until combined.
Cut the chicken into thin strips. Add the chicken to the marinade and marinate in the refrigerator for 30 minutes.
Heat a large skillet over medium-high heat. Add the chicken with marinade and cook, stirring for 15 minutes. The sauce will be reduced during the cooking time. Taste and adjust the seasoning.
Serve chicken warm with rice or veggies garnished with spring onions.
Notes
When you start cooking your chicken, you will have lots of marinade. After cooking you should be left with 2 tablespoons of the sauce.