Slice the beef thinly against the grain. Toss the beef slices with ¼ teaspoon salt, ¼ teaspoon pepper, 1 teaspoon cornstarch and 1 tablespoon water. Then add ½ tablespoon oil and mix well. Set aside to marinate for 15-30 minutes depending on how much time you have.
Add all ingredients for the stir-fry sauce to a bowl and stir well to combine. Set aside.
Cook the egg noodles according to package instructions but undercook for a minute. Then rinse under cold water and drain thoroughly. Set aside.
Place a pan over medium heat. Once it is hot, add some oil and increase the heat to medium-high. Add the beef slices in a single layer and let them cook undisturbed for about 20-30 seconds. Then add half of the minced garlic and stir-fry the beef for a few more seconds until it is 80% cooked and still a bit pink. Transfer to a plate and set aside.
Add some more oil to the pan and adjust the heat to medium. Add julienned carrots and cook for about 30 seconds. Then add onions, stir and cook for about 45 seconds until fragrant. Add the remaining minced garlic and cook for a few more seconds until fragrant.
Turn up the heat to medium-high to high, and add the napa cabbage and baby bok choy. Stir-fry for about 1-2 minutes until the vegetables have wilted but are still crisp.
Add the prepared noodles to the pan and toss with the vegetables briefly, about 30 seconds, until the noodles have warmed up. Add the stir-fry sauce and continue to stir-fry briefly.
Add the beef along with any juices back to the pan. Stir and toss everything together quickly until flavors are combined, beef is cooked through and vegetables are crisp-tender. Adjust seasonings to your taste.
Transfer the stir-fried noodles to a serving plate. Sprinkle black pepper on top and serve immediately.