01 - Combine sliced flank steak with soy sauce, cornstarch, and sherry or rice wine in a medium bowl. Mix thoroughly to coat each piece evenly. Let stand for 10 minutes at room temperature.
02 - Whisk together soy sauce, oyster sauce, brown sugar, cornstarch, broth, sesame oil, and black pepper in a small bowl until the sugar and cornstarch dissolve completely. Set aside near the stove.
03 - Bring a pot of water to a rolling boil. Add broccoli florets and cook for exactly 2 minutes. Drain immediately and rinse under cold running water to halt cooking and preserve vibrant color. Set aside.
04 - Heat 1 tablespoon vegetable oil in a large skillet or wok over high heat until shimmering. Add marinated beef in a single layer and stir-fry for 2 to 3 minutes until evenly browned but still pink in the center. Transfer to a plate.
05 - Add remaining 1 tablespoon oil to the hot pan. Sauté minced garlic and ginger for 30 seconds until fragrant, being careful not to burn. Add blanched broccoli and stir-fry for 1 to 2 minutes until heated through.
06 - Return beef to the pan. Restir the sauce to redistribute cornstarch, then pour over the beef and vegetables. Cook, stirring constantly, for 2 to 3 minutes until the sauce thickens to a glossy consistency and coats everything evenly.
07 - Transfer to a serving platter or plate over steamed jasmine rice. Sprinkle with toasted sesame seeds and sliced scallions if desired. Serve immediately while hot.