
Combine 1 cup vegetable oil and 1 cup all-purpose flour in a large 6 to 8-quart pot, then cook over medium-low heat for 30-45 minutes, whisking occasionally, until the roux is brown.
Increase the heat to medium and add 12 ounces sliced Andouille sausage, 1 ½ cups chopped yellow onion, 1 ½ cups chopped green bell pepper, and 1 cup chopped celery, and cook for about 5 minutes, until the vegetables have softened. Add 2 tablespoons minced garlic and cook for another minute, until fragrant.
Slowly stir in 6 cups chicken broth before adding 3 bay leaves, 2 teaspoons Cajun or Creole seasoning, ¼ teaspoon dried thyme, and salt and ground black pepper to taste. Return the heat to medium-low and simmer uncovered for about an hour.
Add 3 cups shredded cooked chicken and continue to simmer for another 45-60 minutes.
About 15 minutes before serving, stir 1 pound peeled and deveined raw shrimp into the gumbo and simmer until the shrimp is pink and cooked through.
Discard the bay leaves before serving and garnish with 3-4 chopped green onions. Serve with white rice.