Make a roux by cooking shortening and flour together to a golden brown color while stirring constantly. Add onions and cook until soft. Add crawfish fat. Stir constantly while cooking over low heat until shortening floats on top. Add water and seasonings.
Simmer over low heat for 30 minutes. About 1/2 before serving, add garlic and crawfish tails and let simmer for 25 minutes longer. Add parsley, green onion tops, celery leaves and simmer 5 minutes longer. Just before serving add about 3/4 tsp. file' powder if desired. Serve in soup bowls with cooked rice.