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.
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 well combined. Set aside.
03 - Bring a large pot of water to a boil. Add broccoli florets and blanch for 1-2 minutes until bright green and just tender. Drain well 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 not fully cooked through. Transfer beef to a plate.
05 - Add remaining 1 tbsp oil to the pan. Stir-fry garlic and ginger for about 30 seconds until fragrant, being careful not to burn them.
06 - Return beef to the pan, add blanched broccoli, and pour in the sauce mixture. Stir-fry for 2-3 minutes until sauce thickens and coats everything evenly and beef is cooked through.
07 - Transfer to a serving platter and garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice.