In a bowl, combine beef with soy sauce, sugar, salt and pepper. Refrigerate for 30 minutes.
Heat the cooking oil and then sear the marinated beef for 5 minutes.
Pour-in the remaining marinade, water, and sesame oil. Stir and then let boil. Simmer for 20 minutes Add water as necessary.
Let the liquid reduce to half. Pour-in cornstarch diluted in water. Stir until the sauce thickens.
Add the onion. Cover and cook for 5 minutes.
Transfer to a serving plate. Sprinkle sesame seeds on top.
Serve. Share and enjoy!