In a small bowl, combine the sliced flank steak, soy sauce, oyster sauce, and cornstarch. Mix well and let it sit for about 10 minutes.
While the beef is marinating, bring a pot of water to a boil. Add the broccoli florets and blanch for 2-3 minutes until they're bright green and slightly tender. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat until hot.
Add the marinated beef to the skillet in a single layer. Cook for 3-5 minutes, stirring frequently, until browned and cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant.
Return the beef to the skillet, add the blanched broccoli, beef broth, and sesame oil. Stir to combine all ingredients and cook for an additional 2-3 minutes until heated through.
Season with salt and pepper to taste.
Serve the beef and broccoli over cooked rice or noodles.