Preparation:

• Heat the oil in a large, heavy-based stock pot. Add the onions and fry until they start to colour. Add the beef bones and 3 litres cold water.
• Bring to the boil, then turn down the heat, add half the ginger and simmer over a very low heat for at least 8 (and up to 12) hours, semi-covered.
• For the last 2 hours, add the spices and the remaining ginger and simmer. At the end of cooking time, pour the soup through a fine sieve into a clean pan, then season with fish sauce. You will need 400ml for beef stock for this recipe (freeze the rest). Keep the 400ml stock hot.
• Cook the noodles according to the instructions on the packet. Divide between four wide soup bowls and garnish with sprouts, spring onions and basil and coriander leaves. Place the beef slices on top, then pour over the hot stock. 
• Serve with lime the wedges, chopped chillies, more herbs and bean sprouts and additional fish sauce separately.
• Serve with the sriracha chilli sauce in small dipping bowls so you can dip the beef slices in these before eating.