Blanch the udon noodles with hot water for 1-2 minutes to loosen them. Drain and rinse with cold water and set it aside.
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 red pepper and toss it well.
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.
Steak - Use chicken, prawns/shrimp, or pork instead of beef steak.
Vegetables - Other stir fry vegetables can be added or substituted. If you use hard vegetables like broccoli or tender stem broccoli, blanch them in hot water for few minutes before adding to stir fry.
Mirin - Mirin is a Japanese sweet cooking rice wine, similar to sake. If you cannot get mirin, substitute with Chinese Shaoxing wine or dry sherry. For halal or non-alcohol cooking, you can use rice vinegar and a dash of sugar as substitutes.
Adjust sugar - If you prefer your sauce sweeter, add extra sugar or adjust to your preference.
