Char onion and ginger under broiler or over open flame until blackened, about 10 minutes. Rinse off charred bits.
Toast star anise, cloves, cinnamon, and coriander seeds in a dry pan over medium heat until fragrant, about 2 minutes.
Place beef bones in a large pot, cover with cold water, bring to a boil, and drain. Rinse bones.
Return bones to pot with 4 quarts water, charred onion and ginger, toasted spices, salt, fish sauce, and sugar. Bring to a boil, then reduce to a simmer. Skim impurities. Simmer for 3 hours.
Strain broth through a fine-mesh sieve. Discard solids. Keep broth hot.
Cook rice noodles according to package directions. Drain and divide among bowls.
Place raw beef slices on top of noodles. Ladle hot broth over beef to cook it.
Serve with bean sprouts, Thai basil, lime wedges, hoisin sauce, and sriracha.
Tips
For richer broth, simmer bones for up to 6 hours.
Slice beef as thinly as possible for quick cooking.
Leftover broth can be refrigerated for up to 5 days or frozen.