Make the Roux
In a large pot over medium heat, combine the vegetable oil and flour. Stir constantly with a whisk until the roux turns a deep brown color, about 15-20 minutes. Be careful not to let it burn.
Add Vegetables
Once the roux is ready, add the onion, bell pepper, celery, and garlic. Cook for 5-7 minutes until softened and fragrant.
Build the Broth
Stir in the diced tomatoes, chicken or seafood broth, Cajun seasoning, smoked paprika, cayenne pepper, bay leaves, salt, and pepper. Bring the mixture to a boil, then reduce the heat and let it simmer for 20-25 minutes.
Cook the Proteins
Add the sausage slices and cook for 5 minutes. Then, add the shrimp, crab, and fish chunks. Let them cook for 5-7 minutes or until the seafood is fully cooked. If using cooked chicken, stir it in during the last few minutes to heat through.
Serve
Remove the bay leaves and ladle the gumbo over bowls of steamed rice. Garnish with fresh parsley and sliced green onions.