Heat the oil in a large soup pot over medium-high heat. When the oil is hot, add the beef, in batches, and lightly brown.
When all of the meat has browned add the red chili flakes, and then readd the meat back into the pot. Add the chicken broth, water, soy sauce and light brown sugar.
Bring the liquid to a boil, cover, reduce heat to low and simmer for 2 hours. I usually skim the "guck" that comes to the surface the first 20 minutes.
When ready to eat, boil the noodles in salted boiling water until tender. Drain.
Add the cooked noodles to the soup, add the salt and garnish with the green onions. (I sometimes add a little bit of garlic powder to the soup while I'm waiting for the noodles to cook.
Serve at once.
If you don't want the noodles to eventually drink up all of the broth, do not add the noodles to the soup. Instead, put the desired amount of noodles in the bottom of each soup bowl and cover with the soup.