01 - Combine chicken stock, pork bones, onion, garlic, salt, and sugar in a large soup pot. Bring to a rolling boil over high heat, then reduce to medium-low and simmer for 30 minutes. Skim off any foam that rises to the surface for a clearer broth.
02 - Stir in fish sauce to taste. Remove and discard the pork bones and onion halves using a strainer or slotted spoon, keeping only the aromatic liquid.
03 - Add peeled shrimp to the simmering broth and cook for 2-3 minutes just until pink and opaque. Remove shrimp promptly with a slotted spoon and set aside on a plate to prevent overcooking.
04 - Gently place crab meat and sliced fish cakes into the seasoned broth. Simmer on low heat for 5-7 minutes to allow the seafood flavors to meld with the base.
05 - Cook banh canh noodles according to package instructions (typically 3-5 minutes in boiling water). Drain thoroughly and rinse lightly with cool water to prevent sticking. Divide noodles evenly among 4 serving bowls.
06 - Arrange cooked shrimp, crab meat, and fish cakes over the bed of noodles in each bowl.
07 - Pour the hot broth generously over the noodles and seafood, ensuring all ingredients are submerged.
08 - Top each bowl with spring onions, fried shallots, cilantro, and sliced chili if using. Serve with lime wedges on the side and finish with fresh ground black pepper. Serve immediately while piping hot.