Cut the chicken tenderloin into bite-sized pieces.
Slice the onion and green onion thinly.
Make the sauce by mixing soy sauce, sugar, minced garlic, sesame oil, and black pepper.
Heat cooking oil in a pan and stir-fry the chicken tenderloin.
When the chicken is half cooked, add the onion and green onion and continue stir-frying.
Add the sauce and stir-fry over high heat until all ingredients are cooked.
Transfer the finished stir-fry to a plate for serving.