Remove the short rib plates. Pull the meat away from the bones. Reserve the meat and return the bones to the pot. Simmer the broth, uncovered, for 4 to 5 hours. Keep an eye on it; as the liquid boils away, add enough fresh water to cover the bones. Meanwhile, slice the beef sirloin against the grain into paper-thin slices, roughly 2 by 5 cm in size. Slice the reserved short rib meat paper-thin. Set aside. In a small bowl, combine the scallions, coriander and half of the slice onions. Place the remaining sliced onions in a small bowl and stir in the hot chili sauce. Blend well. Soak the rice sticks in warm water for 30 minutes. Drain and set aside. When the broth is ready, remove and discard all of the bones. Strain the broth through a strainer or colander lined with a double layer of dampened cheesecloth into a clean pot. Add the fish sauce and bring the broth to a boil. Reduce the heat and keep the broth at a bare simmer. In another pot, bring 4 litre of water to a boil. Drain the noodles, then drop them in the boiling water. Drain immediately. Divide the noodles among 4 large soup bowls. Top the noodles with the sliced meats. bring the broth to a rolling boil. Ladle the broth directly over the meat in each bowl (the boiling broth will cook the raw beef instantly). Garnish with the scallion mixture and freshly ground black pepper. Serve the onions in hot chili sauce and the accompaniments on the side. Each diner will add these ingredients as desired. Yield: 4 servings.