In Vietnam, a bowl of this noodle soup -- or one of its variations -- is often served for breakfast.
Wrap beef in plastic wrap, and place in the freezer for about 1 hour.
Meanwhile, fill a large bowl with cold water; add rice noodles. Let soak until soft, about 30 minutes. Drain; set aside.
Combine beef stock, 1 1/2 cups water, and fish sauce in a large saucepan, and set over medium-high heat.
Remove beef from freezer. Using a mandoline or a very sharp knife, slice beef into very thin (1/8 inch) strips.
Divide noodles among six serving bowls, and arrange strips of meat over them. Raise heat under stock, and bring to a boil. Pour boiling stock over meat and noodles, pushing beef into hot liquid so that it cooks. Arrange bean sprouts and other accompaniments over beef and noodles; serve immediately with Asian chile sauce, if desired.