Thinly slice the beef against the grain and season lightly with black pepper.
Heat vegetable oil in a wok or large skillet over high heat.
Sear the beef for 2–3 minutes until browned, then remove and set aside.
Add the sliced onions and minced garlic, stir-frying until the onions become soft and lightly caramelized.
Return the beef to the pan and add soy sauce and oyster sauce.
Toss everything together for 2–3 minutes until the beef is fully coated in the savory sauce.
Serve hot with steamed rice or noodles, garnished with sliced green onions if desired.