Take the flank steak slices and cut them into narrow strips. Add to a medium bowl, along with the cornstarch, water, light soy sauce, salt, and white pepper. Mix well, then toss in the oil. Marinate for 15 minutes on the counter.
Meanwhile, prepare the ginger, scallion, daikon, and cilantro.
Add oil to a soup pot over medium-high heat. Add the ginger and scallion, and cook for 1 minute. Increase the heat to high, and add the daikon. Cook, stirring, for 2-3 minutes.
Add the water, salt, white pepper, and oyster sauce. Stir and bring to a boil. Boil for 2-3 minutes, or until the daikon turns translucent.
Now add the glass noodles and beef, gently stirring to separate the beef pieces. Bring back up to a boil, and skim off any foam floating on top of the soup. Add the cilantro and salt to taste. Serve immediately.