In a small bowl, combine soy sauce, oyster sauce, sesame oil, and cornstarch. Stir until smooth.

Place the sliced beef in a mixing bowl and pour half of the sauce over it. Mix to coat evenly and let marinate for 10 minutes.

Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for 2-3 minutes until browned. Remove the beef from the skillet and set aside.

In the same skillet, add the remaining tablespoon of vegetable oil. Add minced garlic and ginger, and stir-fry for 30 seconds until fragrant.

Add the onion, broccoli, carrot, and bell pepper to the skillet. Stir-fry for 3-4 minutes until the vegetables are slightly tender but still crisp.

Return the beef to the skillet, along with the remaining sauce and 2 tablespoons of water. Stir everything together and cook for another 2-3 minutes until the beef is fully cooked and the sauce has thickened.

Season with salt and pepper to taste. Serve immediately over steamed rice or noodles, and garnish with sesame seeds and chopped green onions if desired.