Prepare the beef: Slice the beef thinly against the grain. In a medium bowl, whisk together the marinade ingredients. Add beef slices and toss to coat. Marinate for at least 20 minutes or up to 1 hour in the refrigerator.

Make the sauce: In a small bowl, combine oyster sauce, soy sauce, brown sugar, beef broth, and sesame oil. Stir until sugar dissolves and set aside.

Blanch the broccoli: Bring a pot of water to boil. Add broccoli and cook for 1 minute. Drain and immediately rinse with cold water to stop the cooking process.

Cook the beef: Heat 1 tablespoon of oil in a wok or large skillet over high heat until almost smoking. Add the marinated beef in a single layer and sear for 1 minute without stirring. Then stir-fry for another 1-2 minutes until beef is nearly cooked through but still slightly pink in the center. Remove beef to a plate.

Stir-fry aromatics: Add remaining oil to the wok. Add onion and stir-fry for 1 minute. Add garlic and ginger, stir-frying for 30 seconds until fragrant.

Combine everything: Return beef to the wok along with blanched broccoli. Pour in the prepared sauce and toss everything together for 1-2 minutes until the sauce thickens and coats everything evenly.

Finish and serve: Sprinkle with sesame seeds and sliced green onions. Serve immediately over steamed rice.