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. Add the 18 cups of water and bring to a boil.
Meanwhile, heat the oil in a small nonstick skillet. Add the onion and ginger, cut sides down, and cook over moderately high heat until richly browned.
Add the onion and ginger to the pot along with the bay leaves, cinnamon sticks, rock sugar and 1 tablespoon of kosher salt. Put the cloves, star anise and fennel seeds in a tea ball or tie them up in a piece of cheesecloth. Add them to the pot and simmer, skimming occasionally, until the oxtails are tender, about 2 hours.
Strain the broth in a large sieve set over a heatproof bowl.
Remove the meat from the oxtails and chill.
Put the rice vermicelli in a large bowl and cover with cold water. Let the vermicelli soak until pliable, about 20 minutes.
Bring the broth to a simmer. Place the thinly sliced raw beef in a large strainer and lower it into the broth for 4 seconds.
Working in 6 batches, put the vermicelli in the strainer and lower it into a pot of boiling water for 30 seconds, or until the vermicelli is barely tender.
Transfer the sliced beef and vermicelli to soup bowls. Ladle about 1 1/2 cups of the broth over each bowl of vermicelli and add the chilled oxtail meat.
Serve the soup with the condiments and the sesame-chile oil.
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