In a large pot, heat the vegetable oil over medium heat. Gradually whisk in the flour, stirring constantly until the mixture turns a dark brown color, about 20-30 minutes.

Be careful not to burn the roux; adjust the heat as necessary.

Add the chopped onion, bell pepper, celery, and garlic to the roux. Cook for about 5-7 minutes until the vegetables are softened.

Pour in the chicken broth and diced tomatoes. Stir in the cajun seasoning, thyme, bay leaves, salt, and black pepper. Bring to a boil.

Reduce the heat to low and let the gumbo simmer for 45 minutes, stirring occasionally.

Add the shrimp, crab meat, and andouille sausage to the pot. Cook for an additional 10-15 minutes until the shrimp are pink and cooked through.

Remove the bay leaves and serve the gumbo hot, garnished with chopped green onions and parsley.