Roux

Heat oil in a heavy cast iron dutch oven until it shimmers.
Add the flour and stir to incorporate, breaking up any lumps. Cook until the roux is a dark brown color, stirring and scraping the bottom of the pan constantly.
If you don't have experience making roux, do this over low heat; it may take as long as half an hour. Take care not to burn.

Gumbo

Add the onions to the roux and stir to incorporate; cook, stirring frequently, until the onions caramelize. 
Add the remaining vegetables except the okra and stir to incorporate; cook for two minutes. Add one cup of stock and stir to thin roux and vegetables.  Add remaining stock, sausages, okra, gumbo crabs, about 1/2 pound shrimp, and spices.
Bring to a boil, reduce to a simmer, and cook for about 2 hours until the okra disappears. Stir it often, scraping the bottom and sides of the pot each time.
When it’s done, add the remaining shrimp and the crab meat, simmer for five minutes, remove from the heat and let the pot rest for 20 minutes.

Serve over hot steamed rice with saltines and your favorite Louisiana-style hot sauce.