In a small bowl, mix soy sauce, oyster sauce, cornstarch, water, sesame oil, black pepper, and red pepper flakes (if using). Set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and cook for about 3-4 minutes until browned, stirring frequently. Remove the beef from the pan and set aside.
In the same pan, add broccoli florets and stir-fry for 2-3 minutes until bright green and tender-crisp. Add minced garlic and ginger, cooking for an additional 1 minute until fragrant.
Return the beef to the pan and pour the sauce mixture over the beef and broccoli. Stir well to combine and cook for another 2-3 minutes until the sauce thickens and everything is heated through.
Serve hot over cooked white rice.