Cut the chicken breast into bite-sized pieces.
Julienne the onion and green onion.
Make the marinade with soy sauce, gochujang (Korean chili paste), sugar, minced garlic, sesame oil, and pepper.
Marinate the chicken breast pieces in the sauce for 20 minutes.
Heat oil in a pan and stir-fry the chicken breast.
Add the onion and green onion, and stir-fry together until the meat is cooked.
Turn off the heat and finish with a sprinkle of sesame seeds.