01 - Combine flank steak with 2 tbsp soy sauce, 1 tbsp cornstarch, and 1 tsp sesame oil in a medium bowl. Toss to coat thoroughly and let marinate for 10–15 minutes while preparing remaining ingredients.
02 - Whisk together 1/4 cup soy sauce, oyster sauce, hoisin sauce, brown sugar, 2 tsp cornstarch, and beef broth in a small bowl until smooth and fully combined. Set aside.
03 - Bring a large pot of water to a rolling boil. Add broccoli florets and cook for 1–2 minutes until bright green and crisp-tender. Drain immediately and set aside.
04 - Heat 1 tbsp vegetable oil in a large skillet or wok over high heat. Add marinated beef in a single layer and sear for 1–2 minutes per side until browned but still slightly pink inside. Transfer beef to a plate.
05 - Add remaining 1 tbsp oil to the pan. Stir-fry garlic and ginger for 30 seconds until fragrant, being careful not to burn.
06 - Return beef to the pan with broccoli. Pour in sauce and stir-fry for 2–3 minutes until sauce thickens and coats everything evenly. Remove from heat immediately.
07 - Garnish with sliced green onions and sesame seeds if desired. Serve hot over steamed rice.