Place an 8-quart stockpot over medium heat, and add the oil. Allow the oil to heat for about 5 minutes, then add the flour to the pot. Stir the oil and flour together with a wooden spoon to form a roux. Lower the heat to medium low and continue to stir the roux for 15 to 20 minutes, or until the color of milk chocolate. Add the onions, bell peppers, and celery to the roux and stir to blend. Stir the vegetables for 5 minutes, then add the garlic. Cook the garlic for 30 seconds before adding the beer and Shrimp Stock to the pot. Season the gumbo with the thyme, bay leaves, gumbo crabs, Worcestershire, salt, and cayenne. Bring the gumbo to a boil and lower the heat to a simmer. Continue to simmer the gumbo for 1 hour, skimming the foam and any oil that rises to the surface.
Season both the shrimp and the catfish with 1 1/2 teaspoons Essence. Stir the shrimp and fish into the gumbo and cook for 2 minutes. Add the oysters to the pot and cook, stirring often, for an additional 5 minutes. Taste the gumbo and season if necessary.
Garnish with the parsley and green onions and serve in shallow bowls over white rice.
Makes 3 quarts, 8 to 10 servings