Cut the chicken tenderloin into bite-sized pieces.
Slice the onion and green onion into thin strips.
Add cooking oil to a pan and stir-fry the chicken tenderloin.
Add the onion and green onion to the chicken and stir-fry together.
Add soy sauce, minced garlic, and black pepper; stir-fry until well seasoned.
Once everything is evenly coated with the seasoning, turn off the heat and plate the dish.