01 - Combine thinly sliced flank steak with 1 tablespoon cornstarch, 1 tablespoon soy sauce, and 1 teaspoon sesame oil in a medium bowl. Mix thoroughly and allow to marinate for at least 10 minutes.
02 - In a small bowl, whisk together 4 tablespoons soy sauce, 2 tablespoons oyster sauce, 1 tablespoon brown sugar, minced garlic, grated ginger, 2 tablespoons water, and 1 teaspoon cornstarch until smooth. Set aside.
03 - Bring a pot of water to a boil. Briefly blanch the broccoli florets for 1 minute until vibrant green and just tender. Drain thoroughly and set aside.
04 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Spread marinated beef in a single layer and sear for 1 to 2 minutes until browned but not fully cooked. Remove beef and set aside.
05 - Add remaining 1 tablespoon vegetable oil to the pan. Stir fry broccoli and optional carrot slices for 2 minutes to combine and warm through.
06 - Return beef to the pan, pour the garlic soy sauce over, and toss to coat all ingredients evenly. Stir fry an additional 2 to 3 minutes until the sauce thickens and beef is cooked through.
07 - Remove from heat and garnish with sliced green onions and toasted sesame seeds before serving.