In a small bowl mix soy sauce, oyster sauce, cornstarch, and water to make the sauce.
Heat vegetable oil in a wok or large skillet over medium-high heat.
Add sliced beef and stir-fry for about 3–4 minutes until browned.
Add sliced onions and cook for 2–3 minutes until slightly softened.
Stir in minced garlic and cook for about 30 seconds.
Pour the prepared sauce into the skillet.
Stir everything together and cook for another 2 minutes until the sauce thickens.
Sprinkle black pepper and mix well.
Garnish with chopped green onions before serving.