Heat oil in a heavy-bottomed pot over medium heat.
Whisk in flour and cook, stirring constantly, to form a dark brown roux (about 20–30 minutes).
Add diced onion, bell pepper and celery; cook until softened.
Stir in garlic and cook 1 minute until fragrant.
Add sliced sausage and brown for 3–4 minutes.
Add chicken pieces and sear briefly to develop color.
Pour in chicken stock slowly, scraping up the roux and browned bits.
Add bay leaves, thyme, smoked paprika, cayenne and tomatoes if using; bring to a simmer.
Add okra and reduce heat; simmer gently for 45–60 minutes until flavors meld.
Adjust salt and pepper; remove bay leaves and stir in file powder off heat if desired. Serve over steamed rice and garnish with parsley and green onions.