In a bowl, soak rice noodles in enough cold water to cover for 30 minutes. Drain.
In a large saucepan, bring the 6 cups water to boiling. Add noodles; boil for 4 to 6 minutes or until tender. (To avoid overcooking noodles, test every minute beginning at 4 minutes.) Drain in a colander; rinse with cold water. Drain again. Cut up any long noodles with scissors for easier serving, if you like.
Cut beef into large, paper-thin slices about 4x3 inches or cut into bite-size pieces. Divide the cooked rice noodles among 4 large soup bowls.
In a saucepan, bring beef broth to just boiling. Add meat and cook for about 1 minute or until meat is just cooked. Ladle broth and meat over noodles in bowls. Sprinkle with green onions and cilantro. Serve as you like with hot-pepper sauce, oyster sauce, basil, crushed red pepper, and bean sprouts. Makes 4 servings.
Look for the unusual ingredients in Asian grocery stores or the Asian food sections of large supermarkets.
Use low-sodium beef broth, if you like.