Slice the beef thinly.
Julienne the onion and green onion.
Make the marinade with soy sauce, sugar, minced garlic, sesame oil, and black pepper.
Add the marinade to the beef and mix well.
Heat cooking oil in a pan and spread the beef out to cook.
Add onion and green onion on top and stir-fry together until crispy.
Transfer to a plate and sprinkle with sesame seeds to finish.