Prep the ingredients: Cut the onion and scallion. Cut the beef into cubes or thin slices, whichever you prefer.
Marinate the beef: Toss the beef with cooking wine, oyster sauce, oil, and white pepper. Let it sit for 10 to 15 minutes if you have time.
Heat the pan: Warm a pan over medium high heat and add a small amount of oil.
Sear the beef: Add the marinated beef and pan fry on all sides for about 3 to 4 minutes until nicely browned. Transfer the beef to a plate and set aside
Cook the aromatics: If needed, add a little more oil to the pan. Add the onion and the white parts of the scallion and cook for about 2 minutes until softened and fragrant.
Return the beef: Put the seared beef back into the pan with the aromatics.
Add the sauce: Pour in the dark soy sauce, soy sauce, water, sugar, and the cornstarch mixed with a splash of water to dissolve. Stir gently to combine.
Simmer with a lid: Cover the pan and cook on medium heat for about 2 minutes to let the flavors come together and finish cooking the beef.
Thicken the sauce: Remove the lid, increase the heat slightly, and cook for about 1 minute while stirring so the sauce thickens and becomes glossy.
Serve: Spoon the scallion beef over hot rice and garnish with the green parts of the scallion. Enjoy!