Marinate Beef: In a bowl, toss the thinly sliced beef with soy sauce and cornstarch until well coated. Set aside to marinate for 10-15 minutes.
Cook Noodles: Cook the lo mein noodles or spaghetti according to the package instructions. Drain and set aside.
Stir-Fry Beef: Heat 1 tbsp of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry until browned and cooked through, about 4-5 minutes. Remove the beef from the skillet and set aside.
Sauté Vegetables: In the same skillet, add the remaining 1 tbsp of vegetable oil. Sauté the onion, garlic, carrots, bell peppers, broccoli, and snow peas until tender-crisp, about 5-7 minutes.
Combine: Return the cooked beef to the skillet with the vegetables.
Make Sauce: In a small bowl, mix together soy sauce, oyster sauce, sesame oil, and beef broth. Pour the sauce mixture over the beef and vegetables, stirring to coat evenly.
Thicken Sauce: Add the cornstarch slurry to the skillet and cook for 1-2 minutes, stirring constantly, until the sauce thickens.
Add Noodles: Add the cooked lo mein noodles or spaghetti and toss everything together until well coated.
Garnish and Serve: Garnish with chopped green onions and sesame seeds, if desired. Serve hot.
