In a medium bowl, combine the sliced beef, soy sauce, and cornstarch. Mix well and set aside to marinate for 10 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated beef to the skillet in a single layer and cook for 2-3 minutes without stirring to sear, then stir-fry until beef is browned and cooked through, about 2 more minutes. Remove beef from skillet and set aside.
In the same skillet, add the remaining 1 tablespoon of vegetable oil.
Add the minced garlic and ginger, stir-fry for 30 seconds until fragrant.
Add the sliced red and yellow bell peppers, julienned carrot, and broccoli florets. Stir-fry for 4-5 minutes until vegetables are crisp-tender.
Return the cooked beef to the skillet with the vegetables.
In a small bowl, whisk together the beef broth, oyster sauce, hoisin sauce, sesame oil, and black pepper.
Pour the sauce over the beef and vegetables in the skillet. Stir well to combine and cook for another 2 minutes until sauce thickens slightly.
Remove from heat and stir in the sliced green onions.
Transfer to a serving dish and sprinkle with toasted sesame seeds before serving.