Remove sausage from casing and crumble into a large, heavy-bottomed pot, preferably cast iron. Cook over medium-high heat until cooked through. Transfer to paper towels. Pour off fat from pot.
In same pot, heat oil over medium-high heat. Whisk in flour and continue to stir constantly with a whisk or wooden spoon until roux turns a deep, dark chocolate brown color. This should take about 30 minute. If you sense that the roux is in danger of burning, reduce heat immediately and continue to stir.
Once roux has reached desired shade, stir in onions, bell pepper and celery and continue to stir about 1 – 2 minutes until the vegetables release their steam. Add salt, cayenne and sausage and continue to cook about 5 minutes.
Add bay leaves and stock and bring to a boil over high heat. Reduce heat and simmer, uncovered, for an hour, stirring occasionally to prevent burning. Skim off any fat that rises to the surface. Add turkey and continue to simmer uncovered for 2 hours.
Adjust seasonings and thin out with more stock if necessary. Just before serving, mix filé powder with a few tablespoons of stock, stir to a smooth consistency and then add to gumbo.
For each serving, place mound of rice in bottom of bowl, ladle gumbo over and top with chopped parsley and green onions.