In a large soup pot, cover the oxtails or short ribs with cold water and bring to a boil over high heat. Drain off the water (Tuan repeated this step several times). Add the 3 quarts water of water and bring to a boil.
Lower the heat and simmer (skimming impurities as they rise to the surface).
Meanwhile, place the whole onions right on the burner of the stove and allow to char slightly (see photos).
Add the onion and ginger to the pot along with the cinnamon sticks, salt, star anise, and fish sauce.
Simmer for 3-6 hours, skimming impurities as they rise to the surface.
Strain the broth in a large sieve set over a heatproof bowl. Discard the onions, ginger, cinnamon sticks, and star anise. Remove the meat from the oxtails and add to the stock. Place the stock and meat in back to the stockpot over medium meat.
Add more fish sauce to taste and bring to a boil.
Add the noodles, sliced beef and whatever garnish you choose to the bowls.