01 - In a large soup pot, combine chicken stock, blanched pork bones, onion halves, smashed garlic, salt, and sugar. Bring to a rolling boil over high heat, then reduce to medium-low heat and let simmer uncovered for 30 minutes. Skim off any foam or impurities that rise to the surface.
02 - Stir in fish sauce to taste. Using a strainer or slotted spoon, remove and discard the pork bones and onion halves from the broth.
03 - Add peeled and deveined shrimp to the simmering broth. Cook for 2-3 minutes just until shrimp turn pink and opaque. Remove shrimp immediately with a slotted spoon and set aside on a plate 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 the temperature at a low simmer to prevent the crab from becoming tough.
05 - While broth simmers, cook banh canh noodles according to package instructions. Fresh noodles typically require 2-3 minutes in boiling water, while frozen noodles may take 4-5 minutes. Drain thoroughly and rinse lightly with cool 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.
07 - Ladle the hot, flavorful broth over the noodles and seafood in each bowl, ensuring all ingredients are submerged.
08 - Top each bowl with spring onions, fried shallots, chopped cilantro, and sliced chili if using. Serve with lime wedges on the side and finish with fresh ground black pepper to taste. Serve immediately while piping hot.