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 taste for seasoning adjustments. Remove and discard the pork bones and onion halves from the broth, keeping the liquid at a gentle simmer.
03 - Add peeled and deveined shrimp to the simmering broth. Cook for 2-3 minutes until just pink and opaque. Immediately remove shrimp with a slotted spoon and set aside to prevent overcooking.
04 - Add crab meat and sliced fish cakes to the broth. Simmer gently for 5-7 minutes to allow the seafood flavors to meld with the broth. Keep heat low to maintain a steady simmer without boiling vigorously.
05 - While broth simmers, cook banh canh noodles according to package instructions (typically 2-3 minutes in boiling water for fresh noodles, slightly longer for frozen). Drain well and rinse briefly with cold water to stop cooking and prevent sticking.
06 - Divide cooked noodles evenly among 4 serving bowls. Arrange cooked shrimp, crab meat, and fish cakes on top of the noodles in an attractive presentation.
07 - Ladle hot broth generously over each bowl, covering the noodles and seafood. Top with sliced spring onions, fried shallots, chopped cilantro, and fresh chili if desired. Serve with lime wedges and fresh black pepper on the side for individual seasoning. Serve immediately while hot.