Bun Bo Hue Dac Biet is a traditional Vietnamese noodle soup dish that originated in the city of Hue. It is made with a spicy beef broth that is seasoned with lemongrass, shrimp paste, and chili oil. The soup is typically served with beef shank, pork knuckle, and sometimes pig's blood.
In a large stockpot, combine the beef bones and pork bones with 10 cups of water and bring to a boil. Reduce the heat and simmer for 30 minutes, skimming off any foam that rises to the surface.
Add the lemongrass, shallots, garlic, ginger, and annatto seeds to the pot and continue to simmer for another 2 hours, until the broth is rich and flavorful.
Remove the beef shank and pork knuckle from the pot and allow to cool slightly before slicing thinly.
Add the fish sauce, salt, and sugar to the broth and taste, adjusting the seasoning as needed.
Cook the rice noodles according to the package instructions and divide among serving bowls.
Top the noodles with the sliced beef shank, pork knuckle, and pork blood cubes.
Ladle the hot broth over the noodles and meat, and top with sliced onions, bean sprouts, chili oil, scallions, and herbs.