In a medium bowl, combine the sliced beef, soy sauce, oyster sauce, and cornstarch. Mix well and set aside for 10 minutes to marinate.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated beef to the skillet and stir-fry for 3-4 minutes until browned and cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining 1 tablespoon of vegetable oil.
Add the minced garlic and grated ginger and sauté for 30 seconds until fragrant.
Add the red bell pepper, green bell pepper, carrot, and broccoli florets to the skillet. Stir-fry for 5-6 minutes until the vegetables are tender-crisp.
Return the cooked beef to the skillet with the vegetables.
Pour in the beef broth and sesame oil, stirring to combine all ingredients.
Cook for an additional 2 minutes, allowing the sauce to thicken slightly.
Remove from heat and garnish with sliced green onions and toasted sesame seeds.
Serve immediately with steamed rice or quinoa if desired.