In a medium bowl, combine the thinly sliced flank steak with 1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and optional baking soda. Mix well to coat the beef evenly. Let it marinate at room temperature for at least 15 minutes while you prepare other ingredients.

In a separate small bowl, whisk together 3 tbsp soy sauce, oyster sauce, rice vinegar, brown sugar, 1 tsp sesame oil, minced garlic, grated ginger, and chicken/beef broth. In an even smaller bowl, whisk 1 tsp cornstarch with 1 tbsp water to create a slurry; set aside.

Bring a large pot of salted water to a rolling boil. Add the dried egg noodles and cook according to package directions until al dente. Drain the noodles and rinse briefly with cold water to prevent sticking, then set aside.

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

Add the remaining 1 tablespoon of vegetable oil to the same skillet/wok. Add the sliced yellow onion, bell pepper, and carrot. Stir-fry for 3-4 minutes until the vegetables are slightly tender-crisp. Add the chopped bok choy and stir-fry for another 1-2 minutes until just wilted.

Return the cooked beef to the skillet with the vegetables. Give the prepared stir-fry sauce a quick whisk, then pour it over the beef and vegetables. Bring to a simmer.

Stir the cornstarch slurry once more, then slowly pour it into the simmering sauce while stirring constantly, until the sauce thickens to your desired consistency. Add the cooked and drained noodles to the skillet. Toss everything together to ensure the noodles, beef, and vegetables are evenly coated with the sauce.

Serve immediately, garnished with chopped green onions and toasted sesame seeds, if desired. Enjoy your delicious Beef Noodles!