In a bowl, combine the sliced flank steak with soy sauce, cornstarch, vegetable oil, and sesame oil. Mix well and let it marinate for 15 minutes.

In a separate bowl, mix together the beef broth, soy sauce, oyster sauce, cornstarch mixed with water, and sugar. Set aside.

Heat a wok over medium-high heat. Add a tablespoon of vegetable oil. Add the broccoli, onion, garlic, and ginger. Stir-fry for about 3-4 minutes until the broccoli is bright green and tender-crisp. Remove from the wok and set aside.

In the same wok, add another tablespoon of oil. Add the marinated beef and stir-fry for about 3-4 minutes until browned and cooked through.

Return the vegetables to the wok with the beef. Pour the sauce over the mixture and stir well. Cook for another 2-3 minutes until the sauce thickens. Serve hot over rice or noodles.