Prepare the Sauce: In a small bowl, whisk together the soy sauce, oyster sauce, sriracha, brown sugar, sesame oil, ground ginger, and minced garlic until well combined. Set aside.
Cook the Noodles: Cook the lo mein or spaghetti noodles according to the package directions. Drain and set aside.
Cook the Beef: Season the sliced flank steak with salt and black pepper. In a spacious skillet or wok, warm 1 tablespoon of sesame oil over medium-high heat. Introduce the sliced beef, allowing it to sear until golden brown, roughly 2-3 minutes on each side. Once cooked, transfer the beef to a plate and set aside.
Sauté the Vegetables: In the same skillet, add the remaining 1 tablespoon of sesame oil. Add the broccoli florets, julienned carrots, and red bell pepper. Sauté for about 4-5 minutes until the vegetables are tender-crisp.
Combine Everything: Return the cooked beef to the skillet with the vegetables. Drizzle the prepared sauce over the beef and veggies, ensuring everything is evenly coated. Add the cooked noodles to the skillet and toss again until well combined and heated through.
Serve: Serve the Beef and Broccoli Lo Mein hot, garnished with sesame seeds if desired. Enjoy your delicious and easy homemade lo mein!