Place a dutch oven or heavy bottomed stockpot over medium high heat. Add the oil and heat until very hot, oil will start to shimmer and have a moiré effect. With a wooden spoon, stir in the flour and cook, stirring constantly until the color of the roux is a deep golden brown.
Add the onion, celery, red, yellow and green bell pepper and garlic to the roux and cook for 3-4 minutes until the vegetables are softened slightly and tender.
Stir in the cajun seasoning and cook for one minute until fragrant.
Add the chicken and sausage and cook for an additional 4-5 minutes until the sausage is fragrant and chicken is cooked on the outside.
Add stock one cup at a time, mixing after each addition, so that its well blended and bring to a boil. Reduce heat to a simmer and cook with the lid slightly askew on the pot for 1 hour.
Stir in the okra and simmer for half an hour with the lid askew on the pot, for steam to escape.
Stir in the shrimp and oysters with their liquor and cook for 5 minutes, just until the seafood is cooked through, but not over-cooked. Remove from heat. Stir in the parsley.
Ladle the gumbo into a bowl and garnish with white rice, scallions, parsley and hot sauce to taste.