Remove the shrimp heads, if necessary. Peel and devein the shrimp and refrigerate the shrimp until needed. Combine the shrimp peels and heads and the reserved onion skin and celery trimmings in a 6- to 8-quart pot. Cover with 9 cups of cold water and bring to a boil over high heat. Reduce the heat to a vigorous simmer and cook, uncovered, for 10 minutes. Strain and reserve. You should have about 2 quarts.

In a 10-inch straight-sided saute pan, heat 1/4 cup of the vegetable oil over medium-high heat until hot. Fry the okra in two batches until it becomes lightly browned on the edges, 3 to 5 minutes per batch (fry undisturbed for the first minute or two until browning begins and then stir once or twice to flip most pieces and brown evenly). With a slotted spoon, transfer each batch of okra to a plate or platter lined with a paper towel.

Heat the remaining 6 tablespoons oil over medium-high heat in a 6-quart Dutch oven. Once it's hot, add the flour and stir constantly with a wooden spoon or heatproof spatula until the roux reaches the color of caramel, about 5 minutes. Add the onion and stir until the roux deepens to a chocolate-brown, 1 to 3 minutes. Add the celery and bell pepper and cook, stirring frequently, until slightly softened, about 5 minutes. Add the shrimp stock, okra, tomatoes, gumbo crabs (if using), thyme, bay leaf, salt, and pepper. Adjust the heat to medium-low or low and simmer uncovered, stirring occasionally, for 45 minutes.

Five minutes before serving, add the shrimp, fresh or pasteurized lump crabmeat (if using), oysters, and scallions. Add hot sauce, salt, and pepper to taste. Serve in large soup bowls over 1/4 cup cooked rice per serving. Pass additional hot sauce at the table.

Make Ahead: Store gumbo in the refrigerator for up to three days and then reheat gently before serving. As with many stews and braises, gumbo tastes better the second day. You can also freeze it for up to eight months. Simply transfer to freezer-safe containers.

Gumbo crabs are small blue crabs that have been cleaned and halved or quartered. They are served in the shell, and you pick out the meat as you eat the gumbo. They're available frozen, usually in 1-pound packages. Ask your fishmonger to get you some if you can't find them in your grocery. Fresh or pasteurized lump crabmeat is a reasonable alternative. Do not use shredded or imitation crabmeat.