01 - Toss the sliced flank steak with soy sauce, cornstarch, sesame oil, and sugar in a bowl until evenly coated. Let rest for 10 minutes to absorb the flavors.
02 - In a small bowl, whisk together soy sauce, oyster sauce, water, cornstarch, brown sugar, rice vinegar, minced garlic, and grated ginger until smooth. Set aside.
03 - Bring a pot of water to a rolling boil. Add the broccoli florets and blanch for 1 to 2 minutes until bright green and just tender. Drain immediately and set aside.
04 - Heat 1 tbsp vegetable oil in a large skillet or wok over high heat. Add half the beef in a single layer and sear for 1 to 2 minutes until browned on the outside. Transfer to a plate and repeat with the remaining oil and beef.
05 - Reduce heat to medium-high. Add the blanched broccoli and sliced bell pepper (if using) to the skillet. Stir-fry for 1 to 2 minutes until heated through and lightly charred.
06 - Return the seared beef to the skillet. Pour in the prepared sauce and toss everything together for 2 to 3 minutes until the sauce thickens and evenly coats the beef and vegetables.
07 - Transfer to a serving dish and garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice, brown rice, or noodles.