PUT GINGER, SHALLOTS AND ONION on a baking sheet; place under a hot broiler until charred. In a stock pot, bring the water, oxtails and beef shank to a boil. Thoroughly skim and discard the scum from the surface of the stock. Drop the charred ingredients, star anise, cinnamon stick and cloves into pot, reduce to low heat; simmer for 2 hours. Remove the meat. Remove and shred the meat from the shank and reserve. Return the bone to the simmering stock. Simmer 1 hour longer. When soup is done, remove and discard bones. Strain and degrease stock; add fish sauce and salt. Keep warm.
In a separate pot, bring 3 quarts water to a boil. Drain noodles and add to boiling water; cook for 1 minute. Drain in a colander. Divide noodles among 8 soup bowls (about 1-to-2-cup-size bowls). Divide and top each bowl of noodles with shredded cooked beef, the raw sirloin steak slices, onion slices and bean sprouts. Ladle about 1 1/4 cups hot soup stock (this will cook the beef) to cover the noodles and beef, top with fresh coriander and green onions. Serve with squeeze of lime and chiles.