Heat 1 tablespoon of oil in a frying pan and add the onions.
Cook, stirring until the onions turn brown on the outside.
Remove and drain.
Slice the remaining onion into paper-thin slices and set aside.
Rinse the bones and place in a stockpot.
Cover with cold water and bring to boil slowly.
Reduce the heat and simmer uncovered (for a clear broth, skim off the foam).
After 10-15 minutes, add the fried onion, ginger, carrots, cinnamon, cardamom, star anise, cloves, garlic and peppercorns. Bring to boil.
Simmer the stock, partially covered, for 6 to 12 hours, skimming regularly. If necessary, add water to keep the bones covered.
Strain the stock, skim off and discard any fat. To serve: Arrange the sliced beef on a platter.
Garnish with reserved white and green onion.
On another platter, arrange the bean sprouts, coriander, chillies and limes.
Meanwhile, plunge the rice sticks in boiling water to heat and drain.
Place equal portions in each soup bowl and cover to keep warm.
Heat the beef stock to boiling point and season with fish sauce and pepper.
Pour into a soup tureen or chafing dish.
At the table, place the soup on a portable warmer to keep hot. Offer each guest a bowl of warm rice noodles. Each diner adds some beef and onion to a bowl and ladles the hot stock over the meat, stirring to cook the meat.
Add the bean sprouts, coriander, chillies and lime to taste.