Step 1: Marinate the Beef

1. Prepare the marinade: In a small bowl, combine 2 tablespoons of soy sauce and 1 tablespoon of cornstarch. Stir until smooth.
2. Coat the beef: Add the thinly sliced beef sirloin into the marinade, making sure the pieces are evenly coated. Let the beef marinate for 10-15 minutes while you prepare the rest of the ingredients.

Step 2: Cook the Noodles

1. Cook the noodles: Bring a large pot of water to a boil and cook the lo mein noodles (or spaghetti) according to the package instructions until al dente. This usually takes about 6-8 minutes.
2. Drain the noodles: Once cooked, drain the noodles and rinse them under cold water to stop the cooking process. Set them aside.

Step 3: Stir-Fry the Beef

1. Heat the oil: In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat.
2. Stir-fry the beef: Add the marinated beef to the skillet and stir-fry for about 4-5 minutes, or until the beef is browned and cooked through. Remove the beef from the skillet and set it aside.

Step 4: Sauté the Vegetables

1. Add the remaining oil: In the same skillet, heat the remaining 1 tablespoon of vegetable oil.
2. Cook the vegetables: Add the sliced onions and garlic first, and sauté for about 1 minute until fragrant. Then, add the sliced carrots, bell peppers, broccoli, and snow peas.
3. Sauté until tender-crisp: Stir-fry the vegetables for about 5-7 minutes, or until they become tender but still retain a bit of crispness.

Step 5: Combine Beef and Vegetables

1. Return the beef: Add the cooked beef back into the skillet with the sautéed vegetables. Stir everything together to ensure the beef is evenly distributed.

Step 6: Make the Sauce

1. Prepare the sauce: In a small bowl, whisk together ¼ cup of soy sauce, 2 tablespoons of oyster sauce, 1 teaspoon of sesame oil, and ¼ cup of beef broth.
2. Add the sauce to the skillet: Pour the sauce mixture over the beef and vegetables, stirring to coat all the ingredients.

Step 7: Thicken the Sauce

1. Add the cornstarch slurry: In a small bowl, mix 1 tablespoon of cornstarch with 1 tablespoon of water to create a slurry.
2. Cook until thickened: Add the cornstarch slurry to the skillet and cook for 1-2 minutes, stirring constantly until the sauce thickens and coats the beef and vegetables.

Step 8: Toss with Noodles

1. Add the noodles: Add the cooked lo mein noodles to the skillet, and toss everything together to combine. Ensure the noodles are evenly coated with the sauce and the beef and vegetables are well mixed.

Step 9: Garnish and Serve

1. Garnish: Sprinkle the dish with chopped green onions and sesame seeds, if desired.
2. Serve hot: Serve the Beef Lo Mein immediately while it’s hot and fresh.