How to Make It
In a large pot, combine the fish heads and bones with the stock, onion and 8 cups of water and bring to a boil. Reduce the heat to low and simmer for 20 minutes, skimming off any foam from the surface. Strain the broth into a large bowl.
Wipe out the pot and return the broth to it. Add the okra, oysters, dried shrimp, garlic, chiles and fish sauce and bring to a simmer. Cook over moderately low heat for 10 minutes, stirring occasionally. Add the snapper fillets, shrimp and palm oil and simmer until the fish and shrimp are just cooked through, about 4 minutes. Stir in the crab and cook for 1 minute, until heated through. Season with salt. Serve with rice.
Dried shrimp are available at Asian markets. Palm oil (also known as dende oil) comes from the fruit of the African oil palm. It adds a rich flavor and red color to this gumbo. Look for it at specialty food shops and on amazon.com.