Marinate the beef: In a bowl, combine the sliced sirloin beef with all the "beef marinade" ingredients listed above. Mix evenly until the beef is evenly coated. Set aside for 20 minutes.
Soften the noodles: If using dried noodles, soak in very hot water (just boiled) for about 10 to 15 minutes until softened but still slightly firm, as they will continue cooking in the pan. If using fresh noodles, soak with hot water for 2 minutes. Then, drain and rinse with cold water to loosen it up.
Make the sauce for the noodles: In a bowl, combine all the sauce ingredients. Set aside.
Heat a large wok or skillet on medium-high or high heat. Add a few generous drizzles of oil. Cook the beef until they get a nice sear. This only takes 1-2 minutes. Remove and set aside.
In the same pan, add more oil and cook the sliced yellow onions and scallions (white parts only) for a few seconds until slightly softened. Add 1 or 2 tablespoons of Chinese cooking wine, give it a quick stir to deglaze and let it evaporate. This will infuse with flavor and release the brown bits.
Toss in the softened noodles (drained). Then, gradually add the prepared sauce and toss to combine until evenly coated and the noodles are loosened up, about 2 minutes. Add the scallions (green parts only) and bean sprouts, and cook for another minute or so.
Then, return the seared beef until incorporated. Note: If you’d like more color for the noodles, just add a little bit more dark soy sauce - a little bit goes a long way.
Serve immediately, and top with extra bean sprouts if desired. Enjoy!