In a bowl, combine the sliced flank steak, 2 tablespoons soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the broccoli, bell pepper, and carrots. Stir-fry for 3-5 minutes until the vegetables are tender-crisp.
Add the garlic, ginger, and green onions to the skillet and stir-fry for another minute until fragrant.
Return the beef to the skillet, then add the cooked chow mein noodles, oyster sauce, 2 tablespoons soy sauce, and sesame oil. Toss everything together and cook for an additional 2-3 minutes until heated through.
Serve hot and enjoy!