Slice the onion and green onion into thin strips.
Cut the beef into bite-sized pieces.
In a bowl, make the marinade with soy sauce, sugar, minced garlic, sesame oil, and pepper.
Add the marinade to the beef and let it sit for 10 minutes.
Heat oil in a pan and sauté the onion and green onion.
Add the marinated beef to the pan and stir-fry.
When the beef is almost cooked, place the stir-fried mixture on top of rice.