In a medium frying pan, brown the sausage over medium heat. Remove from pan and drain on kitchen towels to remove some of the fat. Discard fat in pan.
In a large frying pan, heat vegetable oil over high heat. Brown chicken pieces in hot oil. Turn frequently until golden brown on all sides. Transfer chicken to a dish, leaving oil in pan. Set chicken aside, but keep warm.
Make a roux by whisking flour into the hot vegetable oil. Turn heat down to low. Continue cooking flour and oil mixture, stirring constantly, until it reaches a dark brown colour. This may take 30 to 45 minutes; the darker the roux, the better the final gumbo.
When the roux is a dark brown colour, quickly add the sausage, onion, spring onions, green pepper, parsley and garlic. Cook over low heat until the vegetables are wilted, about 10 minutes, stirring constantly.
Stir in 500ml water and spices. Add chicken parts. Add rest of the water slowly. Bring mixture to the boil and reduce heat. Simmer for about 45 minutes, until chicken is done and tender.
Remove chicken pieces and save for another use. Add prawns to gumbo; cook for about 8 to 10 minutes more. Remove bay leaves. Taste, and adjust seasoning. Serve gumbo in deep bowls. Sprinkle file powder over individual servings and stir in.
File powder is also known as sassafras leaves. It is used as a thickener. You can buy ground sassafras leaves in speciality stores or online.