Place 1 tablespoon olive oil in a large heavy bottom pot and heat over medium heat.
When oil is hot, add sliced Andouille sausage to the pot and sauté until browned.
Remove sausage from the pot. Set aside.
Add 1 cup olive oil to the pot. When oil is hot whisk in 1 cup flour.
Cook over medium heat, whisking continuously for 10-15 minutes until roux turns a deep brown color. The color should resemble chocolate milk.
Add diced celery, diced onion and diced green bell pepper to the pot. Cook until vegetables soften, about 5-10 minutes, stirring occasionally.
Add chicken broth and water to the pot, stir to combine.
Add sliced mushrooms if using.
Add seasonings to the pot; onion powder, garlic powder, Creole seasoning, oregano, paprika, salt and pepper.
Stir to combine. Add reserved browned sausage and shredded chicken or turkey to the pot.
Increase heat and bring mixture to a boil.
Once stew comes to a boil, cover and reduce heat. Simmer over low heat of an hour, stirring occasionally.
Taste and season with additional salt if needed.
If using seafood add it now and simmer stew for another 5 minutes.
To serve, place a mound of white rice in the middle of a shallow bowl. Spoon gumbo over the rice and garnish with sliced green onions.