Mix the beef marinade ingredients together and add the beef. Let sit for 30-60 minutes.
While beef marinates, mix the stir fry sauce ingredients together and set aside.
Heat 2 tablespoons of oil in a wok over high heat until smoking. Add the beef (cook in 2 batches) and quickly sear until browned on edges. Remove and set aside.
Add remaining 2 tablespoon of oil to the wok (or skillet).
Add the green onions, garlic and ginger and quickly stir fry for 15-30 seconds, stirring often to prevent burning.
Add the noodles and cook for 30 seconds, tossing constantly.
Add the sauce and toss to coat the noodles in the sauce. Add the bean sprouts and cook 20-30 seconds.
Toss in the beef, remove from heat and serve.
Notes
Do not leave the beef in the marinade for longer than 60 minutes as the baking soda will cause the beef to break down too much.
Make sure to cut the flank steak against the grain to ensure it stays tender when cooked. When looking at the meat, you'll see a grain (much like a wood grain) running in a direction. Slice the opposite way of this grain.
Mix your sauce ingredients together and have your aromatics prepped before starting to cook as the process will go quickly and you want your ingredients ready.
Do not overcrowd the pan when cooking the beef or it will not brown on the edges. I recommend cooking in AT LEAST 2 batches.
You can use a Dutch oven or cast iron skillet instead of a wok if you don't have one.
Use tongs to toss the noodles and ensure they get coated with sauce and don't stick to the wok.
The wok should be smoking before you add the oil for the best results. It need to be very hot to prevent the beef from sticking and to provide that smoky wok cooked flavor (wok hei).