Place bones, oxtails and flank steak in a large stock pot. Add SALTED BOILING water to cover and bring to a boil. Better yet, boil water, add salt then add bones. Marinate oxtails and flank steak with fish sauce, sugar, black pepper.
Cook 5 minutes, drain and rinse pot and bones.
Return bones to pot, add 6 quarts COLD WATER and bring to a boil. Skim surface of scum and fat. Stir bones at bottom. After 1 hour of simmering, add oxtails & flank steak.
The ginger root, shallots, and onion need to be roasted. The ginger root until the outer layer peels off easily. The onion until outer layer is semi-burnt. About 3 min. Roast the anise seeds in a pan for 2 min. Add to broth while waiting to boil.
Meanwhile, slice beef sirloin against grain into paper-thin slices, about 2-by-2 inches. Slice flank steak the same way; that is after they are cooked for about 1 hour.
Set aside. In a small bowl, combine scallions, cilantro, and half the sliced onions.
Place remaining onions in another bowl and mix in hot chili sauce. Soak rice noodles in water. When broth is ready, discard bones.
Strain broth through a colander lined with a double layer of damp cheesecloth into a clean pot. Add fish sauce and bring to a boil.
Reduce heat to simmer.
In another pot, bring 4 quarts of water to a boil.