Start by cleaning and blanching the meats: place the Beef bones, Beef shank and Pork hock in a large pot, cover with cold Water and bring to a rolling boil for 5–10 minutes to remove impurities; drain, rinse the bones and meats, and wipe the pot clean.
Char the aromatics: halve and char the Yellow onion and smash then char the Ginger under a broiler or over an open flame until lightly blackened. Trim and bruise the stalks of Lemongrass (smash the lower stalks with the back of a knife).
Build the broth: return the blanched bones and meats to the pot, add fresh Water to cover, then add the charred Yellow onion, Ginger and the bruised Lemongrass. Bring to a gentle simmer, skim any foam, then reduce heat and simmer very gently for 2.5–3 hours (or longer) until the shank is tender and the broth is rich.
While the broth simmers, make the annatto oil: warm Neutral oil in a small saucepan over low heat, add Annatto seeds and gently heat until the oil turns a deep orange-red; strain and reserve the oil and discard the seeds.
Make the flavor paste: in a small pan, heat a tablespoon of the annatto oil and sauté sliced Shallots and minced Garlic until translucent. Add Chili flakes and a spoonful of Fine shrimp paste (mắm ruốc Huế), cooking briefly to tame the raw edge; scoop some of this aromatic mixture into the simmering broth to deepen its flavor.
Season the broth: taste and adjust with Fish sauce, a chunk of Rock sugar (or to taste), and a little Salt; if you want a touch more sweetness, add a pinch of Sugar. Continue simmering on low so flavors meld, skimming fat as desired.
When the meats are tender, remove the Beef shank and Pork hock; reserve the hock whole and slice the shank thinly across the grain. Shred or slice some of the hock meat to add to bowls. Slice the Chả lụa into rounds and warm briefly in the hot broth or pan.
Cook the noodles: prepare the Dried thick round rice vermicelli (bún bò Huế noodles) according to package directions (usually a short soak or quick blanch), then drain and divide among serving bowls.
Prepare the garnishes: thinly slice White onion, chop Scallions, Cilantro and Sawtooth herb (ngò gai). Rinse and thinly shred the Banana blossom and soak briefly in cold water (and a squeeze of Limes if desired) to keep it crisp and pale. Slice Fresh red chilies and have Limes wedges and ground Black pepper ready for the table.
Assemble each bowl: place a nest of noodles, top with slices of beef shank, a few pieces of pork hock, and rounds of chả lụa. Ladle the very hot broth over the bowl, drizzle a little annatto oil and a spoonful of the sautéed shrimp-paste mixture for color and heat, then garnish with scallions, cilantro, sawtooth herb, white onion, banana blossom and fresh chilies. Serve with extra lime wedges, fish sauce or chili on the side and ground black pepper to taste.
