01 - In a large soup pot, combine chicken stock, blanched pork bones, halved onion, smashed garlic, salt, and sugar. Bring to a rolling boil over high heat, then reduce to medium-low heat and simmer for 30 minutes, skimming off any foam that rises to the surface.
02 - Stir in fish sauce and let the broth simmer for another 2 minutes. Using a strainer or slotted spoon, remove and discard the pork bones and onion halves from the broth.
03 - Add peeled and deveined shrimp to the simmering broth. Cook for 2-3 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 over low heat for 5-7 minutes to allow flavors to meld together. Keep the broth hot.
05 - While broth simmers, cook banh canh noodles according to package instructions (typically boil for 3-5 minutes). Drain well and rinse lightly with cold water to prevent sticking.
06 - Divide cooked noodles evenly among 4 serving bowls. Arrange the cooked shrimp, crab meat, and fish cakes on top of the noodles.
07 - Ladle hot broth generously over each bowl to cover the noodles and seafood. Top with sliced spring onions, fried shallots, chopped cilantro, and sliced red chili if using. Serve immediately with lime wedges and fresh black pepper on the side.