01 - In a large soup pot, combine chicken stock, pork bones, onion halves, smashed 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 during simmering.
02 - Add fish sauce to the broth and stir 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 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 for 5-7 minutes to allow flavors to meld and seafood to heat through.
05 - While broth simmers, cook banh canh noodles according to package instructions (typically 3-5 minutes in boiling water). Drain well and rinse lightly with cold water to prevent sticking.
06 - Divide cooked noodles evenly among 4 serving bowls. Arrange cooked shrimp, crab meat, and fish cakes on top of the noodles.
07 - Ladle hot broth generously over each bowl, covering the noodles and seafood. Top with spring onions, fried shallots, chopped cilantro, sliced chili if using, and a grinding of fresh black pepper. Serve immediately with lime wedges on the side.