To make stock, pour water into a large pot and add skin bones and gravy beef. Bring to the boil. Skim off foaming scum from surface. Turn heat to medium-low, partly cover and simmer for 2 hours, skimming often. Add remaining stock ingredients. Simmer another 90 minutes and remove from heat.
Drain stock through a fine sieve, then set stock aside. Discard bones, carrots, onion and spices. Skim fat from stock once cool. Cut gravy beef finely across the grain. Slice steak to paper thin slices and set aside.
Soak rice noodles in warm water for about 20 minutes until soft. Drain and set noodles aside.
Set stock to boil with fish sauce then reduce heat to very low. Fill a separate large pot three-quarters full of water and bring to the boil. Add dried noodles and washed bean sprouts. Continue boiling until noodles are tender but not mushy. Bean sprouts should retain some crispness.
Pour boiling stock into 6 serving bowls, add drained noodles, then top equally with gravy beef, raw onion rings, chopped spring onions and raw steak slices, and garnish with coriander and mint leaves.
