Cut the beef sirloin into bite-sized pieces.
Prepare and slice the onion, green onion, and garlic.
Make a marinade with soy sauce, sugar, sesame oil, minced garlic, and ground black pepper.
Add the marinade to the prepared beef and mix well.
Add the onion and green onion, then mix everything evenly.
Heat cooking oil in a pan.
Add the marinated beef and vegetables to the pan, and stir-fry on high heat.
Once the beef is cooked through and the marinade is absorbed, remove from heat and serve on a plate.