01 - In a large soup pot, combine chicken stock, blanched pork bones, halved onion, smashed garlic, salt, and sugar. Bring to a boil over high heat, then reduce heat to low and simmer for 30 minutes. Skim off any foam that rises to the surface.
02 - Add fish sauce to the simmering broth, stirring to incorporate. Remove and discard the pork bones and onion halves using a strainer or slotted spoon.
03 - Add peeled and deveined shrimp to the seasoned broth. Simmer for 2 to 3 minutes until just pink and opaque. Remove shrimp with a slotted spoon and set aside on a plate.
04 - Add crab meat and sliced fish cakes to the broth. Simmer gently for 5 to 7 minutes to allow flavors to meld together. Keep the broth warm over low heat.
05 - While broth simmers, cook banh canh noodles according to package instructions. Drain thoroughly and rinse lightly with cool water to prevent sticking. Divide noodles evenly among 4 serving bowls.
06 - Top each bowl of noodles with reserved shrimp, crab meat, and fish cakes. Ladle hot broth generously over the noodles and seafood, ensuring all ingredients are submerged.
07 - Finish each bowl with sliced spring onions, fried shallots, chopped cilantro, and red chili slices if using. Serve with lime wedges on the side and fresh ground black pepper for individual seasoning. Serve immediately while piping hot.