Heat oven to 350°F. Whisk together oil and flour in a cast iron skillet on medium heat for 5 minutes. Put the pot into the oven. Set a timer to check the roux and whisk every 20 minutes until you reach a rich chocolate brown color. Should take about 40 minutes.
Once your roux is done, in a Dutch oven and over medium heat, add in your trinity mix, onion powder, gumbo file, garlic powder, and Cajun seasoning. Cook down for about 20 minutes until the onions are translucent.
Add in tomato paste and roux. Let them cook together.
Add in cubed chicken, and let it cook fully through. Then add in your sausage and let it all cook together for 20-30 minutes.
Add in chicken broth, bay leaves, fresh thyme, and rosemary. Bring it to a boil and cook on low-medium heat for at least 3 hours.
Serve over rice and enjoy!