Marinate the beef: In a bowl, combine the thinly sliced beef, soy sauce, and cornstarch. Mix well and let it sit for at least 15 minutes.
Cook the noodles: Bring a pot of water to boil. Add the lo mein noodles or spaghetti, and cook according to the package instructions until al dente. Drain and set aside.
Heat the oil: 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.
Stir-fry the vegetables: In the same skillet, add another tablespoon of vegetable oil. Add the mixed vegetables, garlic, and ginger, and stir-fry until the vegetables are tender but still crisp, about 3-5 minutes.
Combine everything: Return the beef to the skillet with the vegetables. Add in the cooked noodles, oyster sauce, soy sauce, and sesame oil. Toss everything together gently to mix well and heat through.
Garnish and serve: Stir in the chopped green onions and serve the Beef Lo Mein hot.