Blanch the beef bones and beef shank in a large pot of boiling water for 5 minutes, then drain and rinse under cold water. This removes impurities and ensures a cleaner broth.

Return the blanched bones and shank to the pot. Add 10 cups of fresh cold water, the bruised lemongrass pieces, halved onion, and smashed garlic. Bring to a boil over high heat, then reduce to a gentle simmer.

Skim any foam that rises to the surface during the first 15–20 minutes of simmering. Continue to simmer uncovered for 2 to 2.5 hours, until the beef shank is fully tender and pulls apart easily.

While the broth simmers, make the annatto chili oil: heat the neutral oil in a small saucepan over medium-low heat. Add the annatto seeds and cook, stirring, for 2 minutes until the oil turns a deep reddish-orange. Remove and discard the seeds. Add the chili flakes to the colored oil, stir for 30 seconds off heat, and set aside.

When the beef shank is tender, remove it from the pot and let it cool slightly. Slice it thinly across the grain and set aside. Strain the broth through a fine-mesh strainer and discard the solids.

Return the strained broth to the pot over medium heat. Stir in the dissolved mắm ruốc, fish sauce, sugar, and salt. Pour in all of the annatto chili oil. Taste and adjust seasoning — the broth should be savory, spicy, and faintly tangy. Simmer gently for 10 more minutes.

Cook the bún noodles according to package directions. Drain and divide among four deep bowls.

Top each bowl with sliced beef shank. Ladle the hot broth generously over the noodles and beef.

Arrange the shredded banana blossom, bean sprouts, fresh basil, lime wedges, and sliced chilies on a plate and serve alongside so everyone can garnish their own bowl at the table.