Remove crawfish tails from bag and place in a bowl. Season crawfish with salt and red cayenne pepper. Set aside.
In a heavy-bottomed, 8 quart Dutch oven, heat oil over medium-high heat.
Whisk in flour, STIRRING CONSTANTLY, until dark brown roux is achieved. (Your finished roux color should be the color of cocoa or almonds)
Add onions, bell peppers, and garlic. Sauté 3-5 minutes or until vegetables are wilted.
Pour in tomato sauce and cook 3 minutes.
Slowly add water, 1 cup at a time, stirring well after each addition and allow to come to a low boil before adding next cup of water. This will allow the roux to absorb the liquid and create a smooth stew.
Continue adding all water until a thick stew consistency is achieved.
Add crawfish fat, if using.
Bring stew to a rolling boil then reduce to simmer.
Add crawfish tails and simmer stew for 30-45 minutes, stirring occasionally. Do not boil. Add more water if needed.
Once stew is cooked, taste and adjust seasonings as needed.
Add green onions, parsley, and optional crawfish claws.
Cook 10 minutes.
Serve with cooked white rice.
Adapted by Debra Taghehchian based on a recipe found in The Encyclopedia of Cajun & Creole Cuisine p. 409