Preheat the grill to high and grill the whole onion and ginger until caramelised and deep brown in colour, turning regularly. Cut the beef shank into chunky pieces and place in a large pan with the bones. Add 3.5 litres (6 pints) of water with the charred onion and ginger. Tip in the garlic, cinnamon and star anise. Bring to the boil, then reduce the heat and simmer gently for 2 ½-3 hours until the meat is tender, occasionally skimming off any fat. Leave to cool. This can be done up to two days in advance which allows the flavours to infuse and develop.
Remove the beef shank from the stock and shred into small pieces, discarding any fat or gristle. Strain the stock, discarding the bones and return to the pan with the shredded beef. Add the fish sauce and lime juice.
Slice the fillet steak into wafer thin slices and set aside. Pour boiling water over the noodles and leave to stand for 10 minutes, then drain well and divide among warmed Oriental bowls. Scatter the thinly sliced fillet steak on top. Serve with the bean sprouts, spring onions, Asian basil or coriander, chilli and lime wedges on the side and allow people to help themselves.