Slice flank steak into thin strips against the grain. In a bowl, combine the beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, and sesame oil. Mix well and let sit for 10 minutes while you prep everything else. This tenderizes the meat and creates that velvety texture.

In a small bowl, whisk together 2 tablespoons soy sauce, oyster sauce, brown sugar, rice vinegar, and water or broth. Mix remaining 1 tablespoon cornstarch with 2 tablespoons water to make a slurry and add it to the sauce. Set aside.

Heat your wok or large skillet over high heat until smoking hot. Add 2 tablespoons vegetable oil and swirl to coat. Add the marinated beef in a single layer. Let it sit for 30 seconds without touching so it gets a nice sear. Flip and cook another minute until just done. Remove beef to a plate.

Add remaining 1 tablespoon oil to the same pan. Add broccoli and stir fry for 2 minutes. Add a splash of water and cover for 1 minute to steam. Add garlic and ginger, stir fry for 30 seconds until fragrant.

Return the beef to the pan with the broccoli. Pour in the sauce and toss everything together. Cook for 1 to 2 minutes until the sauce thickens and coats everything. The sauce should be glossy and cling to the beef and broccoli. Garnish with green onions and sesame seeds if using. Serve immediately over steamed rice.