How to Make It
In a large pot, combine all of the ingredients except the crabs and bring to a boil. Add the crabs and simmer briskly over moderate heat for 1 hour, skimming as necessary. Strain the stock into a pot. You should have about 12 cups; add water if necessary, and reserve any extra stock for another use. Discard the crabs.
In a large pot, heat the oil. Whisk in the flour and cook over moderately low heat, whisking constantly, until the roux is deep mahogany brown with a nutty aroma, about 40 minutes.
Stir in the andouille, onion, celery, red and green bell peppers, garlic and okra and cook, stirring occasionally, until the vegetables soften, about 10 minutes. Add the tomatoes, filé powder, Creole seasoning, thyme, bay leaf, salt and the 12 cups of crab stock. Simmer over moderate heat for 1 hour and 30 minutes, stirring occasionally.
Stir in the crabmeat, shrimp, oysters and 2 tablespoons of hot sauce and cook until the shrimp are white throughout, about 3 minutes. Discard the thyme sprigs and bay leaf. Ladle the gumbo into bowls and top with rice. Garnish with scallions; serve with hot sauce.