Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and cook for 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the broccoli and stir-fry for about 2-3 minutes until bright green and tender-crisp. Add the garlic and ginger, cooking for an additional minute until fragrant.
In a small bowl, whisk together the soy sauce, oyster sauce, cornstarch, beef broth, sugar, and black pepper. Pour this sauce over the broccoli and stir well.
Return the cooked beef to the skillet and toss everything together until the beef is heated through and the sauce thickens, about 2-3 minutes.
Serve the beef and broccoli stir-fry over cooked rice.