Blanch the udon noodles with hot water for 1-2 minutes to loosen them. Drain and rinse with cold water and set it aside. ( Or prepare by the packet instructions. Make sure not to overcooked as you are stir frying them again. )
Mix all the sauce ingredients in a bowl, whisk and set it aside. Cut the beef into thin slices against the grain. Season with 1 tablespoon of stir fry sauce mix. Gather and prepare the vegetables.
Heat the large pan or wok to medium-high heat, add 2 tablespoon of oil, stir fry the beef slices for 2-3 minutes. Push the beef to one side, add garlic and onion slices and stir for few seconds and follow with the mushroom. Cook for another 1 minute, then add the pak choi and toss it well.
next place the udon in the middle of the pan, pour the sauce mixture in. Cook for another 2 minutes until the sauce and other ingredients combine evenly.
Turn off the heat and sprinkle chopped spring onions. Transfer to serving plate and serve immediately.