To make the roux, combine flour and vegetable oil in a sauce pan over medium low heat. Using a wooden spoon, continuously stir the roux as it cooks. Eventually, it'll turn darker shades of brown. This will take about an hour to achieve the color you see above. Don't leave your roux! It burns very quickly. Once you've achieved the desired color, remove from heat and let sit until it reaches room temperature then store in an airtight container in the fridge to use the next day for gumbo (or if doing all in the same day, you don't need to put it in a container, just let it sit in the pot while you prepare everything else).
To make the gumbo, in a large dutch oven, brown the sausage in 2 tbsp. of vegetable oil. Once the sausage is brown, remove with a slotted spoon and set aside.
Brown the chicken pieces in the same oil and sprinkle the tbsp. of Tony's Cajun seasoning on the chicken. Once chicken is brown, remove with slotted spoon and set aside.
Next, in the same oil, sauté onion, green bell pepper, and celery (they call this the holy trinity in Louisiana) until soft and tender, then add the sausage and chicken back in the pot.
Pour all the roux on top of the mixture and let it warm up with everything. Stir to coat.
Add the 8 cups of chicken broth on top, stirring to get it all combined. Add cayenne pepper and bay leaves, as well as salt and pepper, to taste.
Bring mixture to a boil then let it simmer for 2-3 hours, stirring occasionally.
Half an hour prior to finishing and serving, whisk in file powder then continue to let it simmer.
Serve over bed of white rice.
Store in airtight containers in the fridge or freeze for later.