01 - Toss the sliced flank steak with soy sauce, cornstarch, sesame oil, and sugar until evenly coated. Let rest for 10 minutes to allow the flavors to penetrate and the cornstarch to tenderize the meat.
02 - In a small bowl, whisk together soy sauce, oyster sauce, water, cornstarch, brown sugar, rice vinegar, minced garlic, and grated ginger until smooth and free of lumps. Set aside.
03 - Bring a pot of salted water to a rolling boil. Add broccoli florets and blanch for 1 to 2 minutes until bright green and crisp-tender. Drain immediately and rinse under cold water to halt cooking. Set aside.
04 - Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat until shimmering. Add half the beef in a single layer without overcrowding. Sear for 1 to 2 minutes until browned on the edges but still slightly pink inside. Transfer to a plate and repeat with the remaining oil and beef.
05 - Reduce the heat to medium-high. Add the blanched broccoli and sliced bell pepper (if using) to the skillet. Stir-fry for 1 to 2 minutes, tossing frequently to maintain their crisp texture.
06 - Return the seared beef to the skillet. Pour in the prepared sauce and toss everything together continuously for 2 to 3 minutes, until the sauce thickens to a glossy coating that evenly clings to the beef and vegetables.
07 - Transfer to a serving platter and garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice, brown rice, or noodles.