Prepare the Beef: In a mixing bowl, combine the beef slices with soy sauce, oyster sauce, hoisin sauce, and cornstarch. Mix well to ensure the beef is evenly coated. Let it marinate for about 10 minutes.
Heat the Oil: In a large skillet or wok, heat the vegetable oil over medium-high heat.
Cook the Beef: Add the marinated beef to the skillet and stir fry for about 2-3 minutes until the beef is browned but not fully cooked through. Remove the beef from the skillet and set it aside.
Stir Fry the Vegetables: In the same skillet, add a little more oil if needed. Add the minced garlic and ginger, and stir fry for about 30 seconds until fragrant. Add the bell peppers, carrot, and broccoli, and stir fry for about 3-4 minutes until the vegetables are tender-crisp.
Combine Beef and Vegetables: Return the beef to the skillet and stir well to combine with the vegetables. Cook for an additional 2-3 minutes until the beef is cooked through and the vegetables are tender.
Season and Serve: Add the sesame oil and green onions, and toss to combine. Season with salt and pepper to taste. Garnish with sesame seeds if desired. Serve hot over steamed rice or noodles.