01 - In a large soup pot, combine chicken stock, pork bones, halved onion, smashed garlic, salt, and sugar. Bring to a boil, then reduce heat and simmer for 30 minutes. Skim off any foam that rises to the surface.
02 - Add fish sauce to the simmering broth. Remove the pork bones and onion using a strainer or ladle, discarding them.
03 - Add shrimp to the broth and simmer for 2-3 minutes until just cooked through. 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.
05 - While broth simmers, cook banh canh noodles according to package instructions. Drain and rinse lightly with cold water to prevent sticking.
06 - Divide cooked noodles evenly among serving bowls. Arrange shrimp, crab meat, and fish cakes on top of the noodles.
07 - Ladle hot broth generously over the noodles and seafood in each bowl.
08 - Top each bowl with spring onions, fried shallots, cilantro, and sliced chili if using. Serve with lime wedges and fresh black pepper on the side. Serve immediately while hot.