Prepare the beef: Slice the flank steak thinly against the grain. Toss it with 1 teaspoon cornstarch and 2 tablespoons water or cooking wine. Let it rest for 15 minutes.
Prepare the broccoli: Cut the broccoli into bite-sized florets. Briefly steam or microwave until bright green and slightly tender, then drain well.
Mix the sauce: Whisk together the oyster sauce, soy sauce, water, brown sugar, cornstarch, and sesame oil in a small bowl until smooth.
Sear the beef: Heat 1 tablespoon oil in a large skillet or wok over medium-high to high heat. Cook the beef in batches until browned and nearly cooked through. Transfer to a clean plate.
Cook the aromatics: Add the minced garlic and ginger to the skillet. Stir for 20β30 seconds until fragrant, taking care not to burn them.
Combine the ingredients: Return the beef to the skillet and add the broccoli. Stir the sauce again, pour it into the pan, and toss everything together.
Thicken the sauce: Cook for 1β2 minutes, stirring until the sauce thickens and coats the beef and broccoli. Add a splash of water if needed and ensure the beef is cooked through.
Serve: Transfer to a serving dish and serve hot with steamed white rice or brown rice.