Wash the Bean Sprouts (2 handful) and set aside.
Cut 1/3 of a White Onion (1/3) into slices and set aside.
Wash the Scallion (2 stalk) and cut off the root ends. Cut the green onions into 2 inch segments. For the thicker parts of the green onions (the white parts) cut those in half length wise as well so they are easier to cook and not chunky. Set aside.
Slice the Sesame Oil (1/4 teaspoon) against the grain, and place it in a bowl with the Flank Steak (300 gram), Dark Soy Sauce (1/2 teaspoon), Cooking Wine (1/2 tablespoon), Soy Sauce (1 tablespoon), Water (2 tablespoon), and Corn Starch (1/2 teaspoon). Mix well and set aside to marinate for 15 minutes.
To make the sauce, in a small bowl, combine Soy Sauce (5 tablespoon), Dark Soy Sauce (2 tablespoon) and Granulated Sugar (1 teaspoon) and set aside.
Place the Flat Rice Noodles (750 gram) on a plate and microwave it for 1-2 minutes in 30-40 second intervals. At the 1 minute mark, try to loosen up the noodles with your hands. The noodles on the outer edge of the plate usually will soften up first. Loosen the soft ones first, and then put them in the middle of the plate so the non-softened ones can be on the outer edge of the plate to have a chance to get soft.
Once the noodles are a little soft and separated, set them aside to cool completely before stir frying.
Set the stove to medium high heat and add a bit of oil to a non-stick frying pan or well seasoned wok.
Once the pan is hot, add in the beef and cook it for about 1-2 minutes (No more than 2 minutes). Once it is cooked (it’s okay for it to be medium rare), remove the beef and set aside for later.
Clean out the frying pan/wok and add in a bit of oil. Once the oil is hot add in the white onion and cook for about 1 minute.
Then add in the noodles and the sauce mix at the same time and stir the noodles with the sauce. Use the sauce to soften up the noodles more, and keep moving the noodles around so it doesn’t sit too long against the pan (else it might stick to it). Cook it for about 1-2 minutes or until it looks soft.
Throw in the bean sprouts and green onions and the cooked beef and quickly toss it into the noodles and turn off the heat.
Continue to toss everything together and let the bean sprouts and green onions cook with the residual heat from the pan. Remove from pan, serve and enjoy!