Start by combining the light soy sauce, vinegar, sugar, corn starch and olive oil with the flank steak and marinate for 15 minutes in the fridge.
Over high heat saute the marinated steak for 5 minutes or until pink is starting to dissipate. Steak is done when internal temperature reaches 165F. Remove steak from pan and save for later.
After lowering the heat, add fresh noodles to the pan with the left-over steak juices and cook for 2-3 minutes until noodles begin to soften.
Stir in the sliced onions and soy bean sprouts to the pan and cook for another 2 minutes.
Add the remaining vinegar, sugar, ginger, oil and both soy sauces. Cook for another 3 minutes.
Gently toss back the flank steak to the noodles, along with the Chinese chives and pepper. You may also add salt and red pepper flakes if desired. Simmer for another 5 minutes and serve warm.
Can be kept in an air tight container up to 4 days in fridge.
Reheat by stove top for 4-5 minutes over medium heat or microwave for 60-90 seconds.
Noodles may be fragile so gently fold when adding ingredients to them in the pan.