In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch. Add thinly sliced beef and marinate for at least 15 minutes.
Heat vegetable oil in a large skillet over medium-high heat. Add minced garlic and ginger, cooking for about 30 seconds until fragrant.
Add the marinated beef to the skillet in a single layer. Sear for 1-2 minutes, then stir-fry for another minute until browned but not fully cooked. Set aside.
In the same skillet, stir-fry sliced green bell pepper, red bell pepper, and onion for 2-3 minutes until tender-crisp.
Return beef to the skillet, add black pepper and sesame oil. Stir to combine and cook for an additional 2 minutes.
Garnish with green onions and sesame seeds, and serve hot over steamed rice.
Notes
Slice beef against the grain for maximum tenderness. Don't overcrowd the pan when cooking the beef.