Toss beef with cornstarch, salt, and pepper. Let it rest for 10 minutes.
Heat oil in a pan and stir-fry beef until browned. Remove and set aside.
In the same pan, sauté onions until soft and slightly caramelized.
Add garlic and ginger, cook until fragrant.
Return beef to the pan and mix well.
Add soy sauce, oyster sauce, dark soy sauce, and brown sugar. Stir until glossy and coated.
Simmer for 2–3 minutes, finish with sesame oil.
Garnish and serve hot with rice.
