Toss sliced beef with cornstarch until lightly coated.
In a bowl, mix soy sauce, oyster sauce, black pepper, sugar, and beef broth.
Heat vegetable oil in a wok or large skillet over high heat.
Stir-fry beef for 2–3 minutes until browned and remove from pan.
Add onion and bell pepper and stir-fry for 2–3 minutes.
Add garlic and ginger and cook for 30 seconds until fragrant.
Return beef to the pan.
Pour sauce mixture over beef and vegetables.
Cook for 2–3 minutes until sauce thickens and coats everything evenly.
Drizzle sesame oil and stir gently before serving.