In a small bowl, whisk together beef broth, soy sauce, oyster sauce, cornstarch, and sesame oil until smooth. Set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the thinly sliced flank steak and stir-fry until browned, about 2-3 minutes. Remove beef from skillet and set aside.
Add broccoli florets to the skillet and stir-fry for 2-3 minutes until they turn bright green and slightly tender-crisp.
Add minced garlic and grated ginger to the skillet and cook for about 30 seconds until fragrant.
Return the cooked beef to the skillet. Pour the prepared sauce over the beef and broccoli. Stir constantly until the sauce thickens and coats everything evenly.
Serve immediately, ideally over rice.
Notes
For a spicier kick, add a pinch of red pepper flakes when sautéing the garlic and ginger. Ensure your beef is sliced very thinly for quick cooking.