Slice beef flank into thin pieces for marinating.

Combine beef slices with light soy sauce, sugar, cornstarch, and sesame oil.

Marinate the beef for 15-20 minutes.

Prepare the seasoning slurry by mixing light and dark soy sauces, sugar, and salt in a bowl.

Heat a wok over medium heat and add grapeseed oil.

Fry the marinated beef until just underdone and set aside.

Add more oil to the wok and saute sliced onions until soft and translucent.

Add ginger slices and bean sprouts to the wok and cook until semi-soft, then push to the side.

Microwave fresh flat rice noodles for 1-2 minutes to warm and separate them.

Turn the heat to high and add the separated noodles to the wok with the vegetables.

Stir the noodles and vegetables with an upward motion to mix.

Add the seasoning slurry to the wok and mix well.

Taste and adjust seasoning, adding more light soy sauce for saltiness or dark soy sauce for color.

Add the previously cooked beef back into the wok and stir to combine.

Garnish with sliced Chinese chives or spring onion.

Serve immediately.