Slice the flank steak thinly against the grain. In a bowl, toss the sliced beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, and 1 teaspoon sesame oil. Set aside to marinate for at least 10 minutes.

Whisk together all the sauce ingredients in a small bowl: soy sauce, oyster sauce, brown sugar, cornstarch, water, sesame oil, and black pepper. Set aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat until the oil is just smoking. Add the beef in a single layer and sear for 1-2 minutes per side until browned. Do not stir constantly. Work in batches if needed to avoid crowding. Transfer to a plate.

In the same skillet, add the remaining 1 tablespoon of oil. Add the broccoli florets and stir fry for 3-4 minutes until bright green and crisp-tender. Add a splash of water if needed to help steam.

Push the broccoli to the side of the pan. Add the minced garlic and grated ginger to the center and cook for 30 seconds until fragrant.

Return the beef to the skillet. Give the sauce a quick stir and pour it over the beef and broccoli. Toss everything together and cook for 1-2 minutes until the sauce thickens and coats everything in a glossy glaze.

Sprinkle with sesame seeds and serve immediately over steamed white rice.