01 - Toss the sliced flank steak with soy sauce, cornstarch, sesame oil, and sugar in a mixing bowl. Ensure each slice is evenly coated and let rest for 10 minutes at room temperature.
02 - In a small bowl, whisk together the 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. Drop in the broccoli florets and blanch for 1 to 2 minutes until they turn vibrant green and reach a crisp-tender texture. Drain immediately and set aside.
04 - Heat 1 tbsp vegetable oil in a large skillet or wok over high heat until the oil just begins to shimmer. Arrange half the beef slices in a single layer and sear for 1 to 2 minutes without stirring until browned on the edges. 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 to the skillet. Stir-fry for 1 to 2 minutes, tossing frequently to maintain their bright color and slight crunch.
06 - Return the seared beef to the skillet. Pour the prepared sauce over the beef and vegetables, tossing continuously for 2 to 3 minutes until the sauce thickens to a glossy finish and evenly coats every piece.
07 - Transfer to a serving platter and finish with sliced green onions and a sprinkle of sesame seeds. Serve immediately over steamed white rice, brown rice, or noodles.