Heat the oil in a Dutch oven over medium heat. Whisk in the flour and cook, stirring constantly, until the roux becomes the color of dark peanut butter, 20 to 30 minutes.
Add the onion, celery, and bell pepper carefully. Stir until softened, about 5 minutes, then add the garlic for 30 seconds.
Simmer the Gumbo:
Add the chicken and sausage, then slowly stir in the stock. Add the tomatoes, okra, Creole seasoning, thyme, Worcestershire sauce, and salt.
Bring to a gentle simmer and cook uncovered for 60 minutes, stirring occasionally, until the chicken is tender and the broth is rich.
Finish and Serve:
Stir in the shrimp and cook for 5 to 7 minutes, until pink and just cooked through.
Spoon rice into bowls, ladle the gumbo over it, and finish with scallions and parsley.
Notes
A dark roux brings the characteristic flavor, but never leave it unattended while it cooks.
For a seafood-forward version, replace the chicken with crab or add oysters with the shrimp.