In a medium bowl, combine the thinly sliced beef, soy sauce, cornstarch, and sesame oil. Mix well until the beef is evenly coated. Let it marinate for at least 10 minutes while you prepare the other ingredients.

In a separate bowl, whisk together the soy sauce, oyster sauce, hoisin sauce, water, rice vinegar, sesame oil, brown sugar, minced garlic, and grated ginger. Set this sauce aside for later use.

Bring a pot of water to a boil. Add the broccoli florets and blanch for about 2 minutes until they turn bright green and are slightly tender. Drain and immediately transfer the broccoli to a bowl of ice water to halt the cooking process. Drain again and set aside.

In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the marinated beef in a single layer, avoiding overcrowding the pan. Sear the beef for about 2-3 minutes until browned and cooked through, occasionally flipping the pieces for even cooking. Remove the beef from the pan and set aside.

In the same pan, add the blanched broccoli and stir-fry for 2-3 minutes. If desired, add a splash more vegetable oil to help with cooking.

Return the cooked beef to the skillet with the broccoli. Pour the prepared sauce over the beef and broccoli and stir to combine. Cook for another 2-3 minutes until everything is heated through and the sauce thickens slightly.

Remove from heat, sprinkle with sesame seeds and chopped green onions. Serve warm over cooked rice or noodles.