Brown chicken in hot shortening in a large Dutch oven. Remove from Dutch oven; drain well, reserving shortening in Dutch oven.
Add flour to hot shortening; cook over medium heat, stirring constantly, 10 minutes or until roux is the color of a copper penny. Add chopped onion; cover, and cook 10 minutes.
Gradually add water to roux, stirring until well blended. Stir in chicken, salt, and pepper. Simmer, uncovered, 1 1/2 hours, stirring occasionally.
Add green onion tops and parsley; simmer 10 minutes. Stir in oysters, and simmer an additional 5 minutes. Remove from heat; stir in filé powder to thicken gumbo, if desired. Serve gumbo over rice.