First, combine your flour and oil in a very large dutch oven over medium high heat. Cook the roux down, stirring constantly. The roux is going to be time consuming. The color will go from white, to blond, to peanut butter, to milk chocolate, to eventually the dark brown you're looking for, This could take about 45-60 minutes. Be sure to scrape the bottom of the pan as you're stirring. Be sure not to burn your roux though, as your gumbo will taste burnt. Black specs in your roux could mean that its been scorched at the bottom of the pot. Taste it to see and if its burned, you have to throw the whole thing out and start over.
Reduce the heat to medium and add your onion, bell pepper, and celery. Sauté for 6-8 minutes, stirring frequently, to get your veggies tender and translucent. Next, add your minced garlic and cook for an additional 60 seconds or until fragrant.
Slowly add in your chicken broth or stock , seasonings, bay leaves, and stir. Taste and adjust seasoning as needed. You can add salt, pepper, and/or cajun seasoning to taste. Bring your stock to a boil and then reduce to a simmer, uncovered, for 45-50 minutes, stirring occasionally.
In a separate large skillet over medium high heat, add a tablespoon of avocado oil. Add in your sliced andouille sausage and sear on both sides for 2-3 minutes. Once browned, remove from the skillet and add in your cubed chicken thighs and breasts. Cook for about 5-6 minutes or until brown.
Once the gumbo is simmered, add the chicken and sausage. Stir to combine and simmer, covered, for an additional 30 minutes. Remove the lid, add your gumbo file and cook uncovered for an additional 15-20 minutes. If too thick, add more broth. Once thickened, remove the bay leaves and add in your hot sauce.
Serve with white rice and/or potato salad and sprinkle with green onion for garnish. ENJOY!