01 - In a large soup pot, combine chicken stock, pork bones, onion, garlic, salt, and sugar. Bring to a boil over high heat, then reduce heat to medium-low and simmer for 30 minutes. Skim off any foam that rises to the surface for a clear broth.
02 - Stir in fish sauce and continue simmering for 2 minutes. Remove the pork bones and onion halves from the broth using a strainer or slotted spoon. Discard or reserve bones for another use.
03 - Add shrimp to the simmering broth and cook for 2–3 minutes until just pink and opaque. Remove shrimp promptly with a slotted spoon and set aside to prevent overcooking.
04 - Add crab meat and sliced fish cakes to the broth. Simmer gently over medium-low heat for 5–7 minutes to allow the seafood flavors to meld with the broth.
05 - While broth simmers, cook banh canh noodles according to package instructions (typically 3–5 minutes in boiling water). Drain thoroughly and rinse lightly with cold water to prevent sticking. Divide noodles among 4 serving bowls.
06 - Arrange cooked shrimp, crab meat, and fish cakes over the noodles in each bowl, distributing evenly.
07 - Ladle hot broth generously over the noodles and seafood. Top with spring onions, fried shallots, cilantro, sliced chili (if using), and a grinding of black pepper. Serve immediately with lime wedges on the side.