In a heavy bottomed stock pot, add oil and flour over medium heat.
To make a roux: stirring constantly in a figure 8 pattern, brown flour and oil mixture until it is the color of peanut butter or darker. STIR CONSTANTLY or it will burn and the roux is ruined.
Once the roux reaches the desired color add onions and celery with a teaspoon of salt. Let the veggies sweat a little, about 5 minutes.
Add chicken broth, stirring roux while adding broth to prevent lumps. Add salt and pepper to taste. If more liquid is needed, add some water.
Add sausage & chicken breasts or pieces, cover, and cook on medium to low heat until chicken comes off the bone easily, about an hour. While gumbo is cooking prepare rice, I usually cook two cups of rice.
After an hour, add finely chopped parsley to gumbo. Turn off heat and add about a teaspoon of file, this will thicken the gumbo.
Serve over a scoop of rice with some warm crusty french bread.