01 - In a large soup pot, combine chicken stock, pork bones, halved onion, smashed garlic, salt, and sugar. Bring to a boil over high heat, then reduce to medium-low heat and simmer for 30 minutes. Skim off any foam that rises to the surface for a clear broth.
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 until just pink and opaque. Remove shrimp promptly 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 infuse into the broth.
05 - While broth simmers, cook banh canh noodles according to package instructions (typically boil 3–5 minutes for fresh, 5–7 minutes for frozen). Drain well and rinse lightly with cool water to prevent sticking.
06 - Divide the cooked noodles evenly among 4 serving bowls.
07 - Arrange the cooked shrimp, crab meat, and fish cakes over the noodles in each bowl.
08 - Ladle the hot broth generously over the noodles and seafood. Top with spring onions, fried shallots, chopped cilantro, sliced chili (if using), and a grind of black pepper. Serve immediately with lime wedges on the side.