1Place the beef bones, water, onion, ginger, star anise, cinnamon, peppercorns, cloves and coriander seeds in a large saucepan over high heat. Bring to the boil.
3Reduce heat to very low and cook, skimming surface occasionally of any fat with a metal spoon, for 3 hours or until liquid reduces by half. Remove from heat and set aside to cool. Strain through a fine sieve into a clean saucepan. Remove and reserve any meat from the bones and discard remaining solids.
5Place the soup over high heat and bring to a simmer. Add the fish sauce and lime juice and stir to combine. Taste and season with salt, pepper, fish sauce and lime juice.
7Meanwhile, place the noodles in a large heatproof bowl and pour over plenty of boiling water. Set aside for 5 minutes to soak. Drain well. Divide noodles and reserved beef evenly among serving bowls.
9Top with sliced beef. Pour the hot soup evenly among each serving bowl. Top with bean sprouts, green onion, chilli, mint and coriander. Serve immediately with lime wedges, if desired.