Bring a large pot of water to a boil and blanch the ribs for about 10 minutes. You will start to see foam and other scum float up to the top. Drain the ribs in a colander and discard the water. This will help remove some of the impurities from the bones which will result in a clearer broth in the end.
Refill the pot with the 3 liters of water. Add onion, tomato, green onion pieces, ginger, garlic, star anise, Chinese five spice powder, sugar, Shiao Xing wine, both soy sauces, and chili bean paste. Add the ribs back in.
Bring the pot to a boil, and then cover and simmer for about 4 hours or until the rib meat is tender. Skim off the excess fat (oil) and/or foam from the surface during the first 30-60 minutes. If you make this ahead of time, the fat will solidify at the top when the soup has cooled. This makes scooping out the excess fat much easier.
Cut the bok choi in half length-wise or chop in large chunks. Blanch the bok choi (or whatever greens) you've selected for a couple of minutes. Be careful not to over blanch then or they will be mushy and soggy in your soup. Rinse in cold water or cool in an ice bath to stop them from cooking. Set aside.
When the soup is ready, and you're ready to serve, prepare the noodles following the package instructions. Portion out the noodles into individual bowls.
Put some bean sprouts on top of the noodles. Add a few pieces of bok choi and 1-2 pieces of rib. Ladle the broth through a strainer into the bowl. Top with pickled mustard greens, sliced green onion, and sliced fresh chilies (if using)
Serve hot, and slurp your way to noodle heaven!