Cut the beef sirloin into bite-sized pieces.
Finely chop the onion, green onion, and garlic.
Make the marinade by mixing soy sauce, sugar, sesame oil, minced garlic, black pepper, and sesame seeds.
Coat the cut beef with the marinade and let it sit for 10 minutes.
Heat cooking oil in a pan and stir-fry the marinated beef.
Stir-fry over high heat for 3-5 minutes until the meat is cooked through.
Transfer to a plate to serve.