01 - Place beef shank and pork hock in a large stock pot. Cover with water, bring to a boil for 5 minutes, then drain and rinse the meat thoroughly to remove impurities.
02 - Refill the pot with 12 cups water. Add cleaned meat, lemongrass, onion, garlic, and ginger. Bring to a boil, skimming off scum as needed. Lower heat to simmer. Add fish sauce, salt, sugar, rock sugar, and shrimp paste. Simmer for 2 hours until meat is tender.
03 - Remove beef shank and pork hock from broth. Slice into serving pieces. Strain broth through a fine-mesh strainer and discard solids. Season broth to taste.
04 - Heat vegetable oil in a small frying pan. Add annatto seeds and cook for 1-2 minutes until oil turns red, then discard seeds. Add shallots and garlic, sautéing until fragrant. Stir in chili flakes and cook briefly. Set aside.
05 - Prepare rice noodles according to package instructions. Drain and set aside. If using pork blood cubes, simmer in gently boiling water for 10 minutes. Slice beef brisket and Vietnamese sausage.
06 - Place a portion of noodles in each serving bowl. Top with beef shank, pork hock, brisket, pork blood cubes, and sausage slices. Ladle hot broth over the top and drizzle with aromatic chili oil.
07 - Add red onion, lime wedges, bean sprouts, banana blossom, fresh herbs, scallions, and chili slices as desired. Serve immediately with extra fish sauce or shrimp paste on the side.