Heat ½ cup oil in a heavy 5-quart pot or Dutch oven and brown chicken on all sides over moderate heat, turning so pieces brown evenly. Remove chicken and set aside.

Make a roux by blending flour into the remaining oil in the same pot and stirring constantly over moderate heat until roux is a deep coffee brown, adding a little more oil if mixture is dry. Add sausage slices, diced ham, onion, green pepper, and celery. Sauté, stirring, for about 10 minutes. Add browned chicken pieces, scallions, garlic, parsley, thyme, bay leaves, and water to cover, stirring to incorporate the roux into the liquid.

Add salt and black pepper and simmer soup, partly covered, for about 1 hour or until chicken is tender. Remove chicken, trim off skin and bones, and return chicken in large pieces to the soup. If you are using the very spicy rind of the tasso ham, you will probably not need cayenne or Tabasco. Otherwise, add either toward the end of cooking and adjust other seasonings. This soup can be prepared in advance up to this point and stored, covered, in the refrigerator for 2 days.

Bring soup to a simmer. Five minutes before serving, remove from heat and stir in filé powder. Cover and let stand for about 5 minutes, but do not simmer or the file will make the soup too thick. (If you prepare more of this gumbo than you need for one meal, remove the portion to be served and add a proportionate amount of file to that. It is better not to boil soup once the file has been added.) Ladle into wide soup plates with a mound of rice and pieces of chicken.