In a medium bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let it marinate for about 15 minutes.
Cook the lo mein noodles according to package instructions, then drain and set aside.
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 florets and bell pepper, stir-frying for about 3-5 minutes until they are tender-crisp.
Add the minced garlic and ginger to the vegetables and stir-fry for an additional 1 minute until fragrant.
Return the beef to the skillet and add the cooked lo mein noodles, oyster sauce, and sesame oil. Toss everything together until well combined and heated through. Season with salt and pepper to taste.
Serve hot, garnished with chopped green onions.