Bring a large pot of water to a boil. Add sausage, and return to a boil. Remove sausage, and drain, reserving water.
Season chicken with salt and pepper. Heat oil in a large heavy-bottomed stockpot over medium-high heat. Add chicken in batches, and cook until browned; set aside. Make a roux: Sprinkle flour over oil, add 2 tablespoons butter, and cook over medium-high heat, stirring constantly, until dark brown, about 10 minutes. Do not let the roux burn; if it does overcook, discard it and start over with fresh oil, butter, and flour. Remove from heat. Cool roux slightly. Using a paper towel, remove excess oil from top of roux by laying it flat across the top. Return pot to heat. Add 6 tablespoons butter, stirring until melted. Add onions, and cook until translucent and very tender, about 10 minutes. Add peppers, celery, garlic, and parsley stems. Cook, stirring frequently until vegetables are tender, about 10 minutes more. While whisking, slowly add 8 cups hot water. Add chicken and sausage, and bring to a boil. Reduce heat, and simmer for 30 minutes.
Skim fat from the top of the reserved pot of sausage boiling water. Add 4 cups skimmed water, the Worcestershire sauce, salt, and pepper to chicken mixture. Cover, and cook over low heat until chicken is tender, about 1 hour. Just before serving, add scallions and parsley leaves. Taste, and adjust for seasoning. Serve hot over white rice.