Heat the vegetable oil up in a large heavy saucepan on medium high heat.
3 tablespoons vegetable oil
Once the oil is hot add the chicken and cook until it is no longer pink, about 5 to 7 minutes. Remove the chicken from the pan and set aside on a plate. You will add it back in later.
1 to 1½ pounds boneless skinless chicken thighs
Drop the salted butter into the saucepan and let it melt. Whisk in the all-purpose flour with the butter to create a roux (a smooth mixture of the flour and butter).
¾ cup salted butter, ¾ cups all-purpose flour
Lower the stove top temperature to medium or medium-low and then cook the roux, frequently stirring, for 30 minutes to one hour or until it turns dark chocolate brown. (This step is necessary for the classic Gumbo taste and color and the roux changes color depending on the heat and materials used, which is why it may take a long time.)
Once the roux is a dark chocolate brown color, stir in the celery, yellow onion, and green bell pepper.
3 celery stalks, 1 yellow onion, 1 green bell pepper
Season with the Cajun seasoning and garlic salt and then cook until the vegetables become nearly fork tender, about 5 minutes.
1 ½ tablespoons Cajun seasoning, 2 teaspoons garlic salt
Gradually add in the chicken broth 1 cup at a time, stirring constantly and making sure the sauce thickens before adding more. Stop stirring in the chicken broth if the sauce isn’t thickening anymore.
5 cups low sodium chicken broth
Bring the mixture to a simmer, uncovered. If the sauce still has some thickness to it, continue with the next step. If not, cook for 5 to 10 minutes to thicken the sauce a little.
Add in the cooked chicken and andouille sausage and return to a simmer.
12 ounces andouille sausage
Add in the shrimp. Cook until the shrimp curls and turns opaque, about 5 minutes.
1 pound raw large shrimp
Plate and serve warm over a serving of rice (optional) with a sprinkle of green onions as an optional garnish.
2 green onions