In a large bowl, combine the thinly sliced sirloin steak with 1 tbsp soy sauce, cornstarch, 1 tsp sesame oil, baking soda (if using), and black pepper. Toss well to coat. Let it marinate for at least 15 minutes at room temperature while you prepare other ingredients.

In a small bowl, whisk together the hoisin sauce, 2 tbsp soy sauce, rice vinegar, brown sugar, grated ginger, minced garlic, and water (or beef broth). Set aside.

Bring a large pot of salted water to a boil. Add the egg noodles and cook according to package directions until al dente. Drain well and set aside. You can toss them with a tiny bit of oil (1/2 tsp) to prevent sticking if desired.

Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid crowding the pan). Stir-fry for 2-3 minutes until browned and cooked through. Remove the beef from the wok and set aside.

Add the remaining 1 tablespoon of vegetable oil to the same wok. Add the broccoli florets and stir-fry for 2-3 minutes until slightly tender-crisp.

Add the sliced red bell pepper and julienned carrots. Continue to stir-fry for another 2-3 minutes until vegetables are crisp-tender.

Return the cooked beef to the wok with the vegetables. Add the cooked noodles.

Pour the prepared hoisin sauce over the noodles, beef, and vegetables. Toss everything together using tongs, ensuring the noodles and ingredients are evenly coated with the sauce. Cook for 1-2 minutes, stirring constantly, until the sauce has slightly thickened and everything is heated through.

Transfer the Hoisin Beef Noodles to serving bowls. Garnish with sliced green parts of the scallions. Serve immediately.