To prepare broth, heat a large stockpot over medium-high heat. Add oxtail, ginger, and shallots; sauté 8 minutes or until ginger and shallots are slightly charred. Add water and next 8 ingredients (through cinnamon stick); bring to a boil. Reduce heat, and simmer 4 hours. Strain broth through a sieve into a large bowl; discard solids. Return broth to pan, and bring to a boil. Reduce heat to medium, and cook until reduced to 10 cups (about 30 minutes). Skim fat from surface; discard fat. Keep warm.
To prepare remaining ingredients, add sliced onion to broth. Place noodles in a large bowl, and cover with boiling water. Let stand 20 minutes. Drain. Place 1/3 cup bean sprouts in each of 6 soup bowls. Top each serving with 1 1/3 cup noodles and 2 ounces eye-of-round. Carefully ladle 1 2/3 cups boiling broth over each serving (boiling broth will cook the meat). Serve with cilantro, basil, chiles, limes, and hoisin, if desired.
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