Marinate the Beef: In a bowl, combine the thinly sliced flank steak with 1 tablespoon soy sauce and 1 teaspoon cornstarch. Toss to coat and let it marinate for 10–15 minutes while you prepare the other ingredients.

Prepare the Sauce: In a small bowl, whisk together the soy sauce, oyster sauce, sesame oil, cornstarch, brown sugar, and water. Set aside.

Cook the Broccoli: In a large pan, warm up 1 tablespoon of vegetable oil over medium-high heat. Add the broccoli florets and cook for about 3 to 4 minutes, stirring occasionally, until they turn vibrant green and are just tender. Transfer to a plate and set aside.

Cook the Beef: Pour the remaining tablespoon of vegetable oil into the pan. Cook the marinated beef in a single layer for about 2 to 3 minutes, turning occasionally until it's nicely browned and cooked through. Once done, take the beef out of the pan and set aside.

Combine and Simmer: Lower the heat to medium and toss in the garlic and ginger, allowing them to infuse the oil with their aroma. Stir-fry for 30 seconds until fragrant. Return the cooked beef and broccoli to the pan, then pour in the prepared sauce. Toss everything together and let it simmer for 2–3 minutes, until the sauce thickens and coats the beef and broccoli evenly.

Garnish and Serve: Remove the pan from heat and garnish the dish with sesame seeds and sliced green onions, if desired. Serve hot over steamed rice or noodles.