In a heavy-bottomed cast iron Dutch oven, combine flour and oil over medium-low heat to begin the roux.
Whisk the roux constantly for 30 to 45 minutes until it reaches the color of dark chocolate, being careful not to burn it.
Immediately add the onions, bell peppers, and celery to the roux to stop the cooking process; sauté for 8 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Slowly whisk in the seafood stock, ensuring the roux integrates without forming lumps.
Add Cajun seasoning, bay leaves, thyme, and cayenne pepper; bring the mixture to a boil.
Reduce heat to low and simmer uncovered for 60 minutes, skimming any foam or oil that rises to the surface.
In a separate skillet, sauté sliced okra in a tablespoon of oil for 10 minutes to reduce mucilage before adding it to the gumbo pot.
Add the shrimp to the pot and simmer for 5 minutes until they turn pink.
Gently fold in the lump crab meat and the oysters with their liquor; cook for an additional 3 to 5 minutes until the edges of the oysters begin to curl.
Adjust seasoning with salt and pepper to taste; remove bay leaves.
Serve in wide bowls over a scoop of white rice, garnished with green onions and parsley.