Prepare the white onion: cut into thin half moons and place in a jar. Cover with vinegar and ground pepper. Place in the fridge until serving.
Place the halved onion in a heavy saucepan and brown well on both sides.
In a large stockpot, combine all stock ingredients, placing the spice bag at the bottom so it does not float. Cover well with water and simmer up to 7 hours, skimming any foam.
After 4 hours cooking, remove the ribs, pick the meat and refrigerate. When stock is done, strain and refrigerate until using. It is better the next day.
Arrange the coriander, chives, ngo gai, basil, bean sprouts and chillies on a serving plate. Heat the stock to a simmer. Cook the noodles according to packet directions in boiling water with a little vegetable oil.
To Serve: Divide the noodles between bowls and top with rib meat and thinly sliced beef. Ladle over the hot stock. Each person helps themselves to the condiments. Serve with hoisin sauce and lime quarters.