How to Make It
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, about 5 minutes. 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. Refrigerate the broth and the oxtail meat separately overnight.
Heat the vegetable oil in a small saucepan. Add the garlic and cook over moderate heat until golden, about 2 minutes. Add the crushed red pepper and sesame seeds and cook for 1 minute; transfer to a bowl. Stir in the sesame oil and a pinch of salt.
Put the rice vermicelli in a large bowl and cover with cold water. Let the vermicelli soak until pliable, about 20 minutes.
Skim the fat from the surface of the beef broth and discard. Bring the broth to a simmer over moderately high heat. Bring a large saucepan of water to a boil.
Place the thinly sliced raw beef in a large strainer and lower it into the simmering broth for 4 seconds; transfer the meat to 6 soup bowls. Drain the vermicelli. Working in 6 batches, put the vermicelli in the strainer and lower it into the boiling water for 30 seconds, or until the vermicelli is barely tender. Drain and transfer to the bowls. Ladle about 1 1/2 cups of the broth over each bowl of vermicelli and add the chilled oxtail meat.
Put each of the remaining ingredients in seperate bowls or arrange the vegetables and herbs together on a platter. Serve the soup with the condiments and the sesame-chile oil.
Rock sugar comes in large amber crystals and is less sweet than refined granulated white sugar. It is available at Asian markets.