Tomato gumbo with sea bass

1 whole sea bass, about 3 lb (1.5 kg), cleaned, filleted and skinned with skeleton and head reserved
5 tablespoons (3 fl oz/80 ml) Vegetable oil
1/2 cup diced yellow onion
1/4 cup (1 1/2 oz/45g) all-purpose (plain) Flour
2 tablespoons diced spicy sausage
1/2 cup (2 1/2 oz/75 g) diced green bell Pepper (capsicum)
1 Celery stalk, diced
3 Cloves Garlic, sliced
1 teaspoon Salt, plus Salt to taste
1/2 teaspoon whole peppercorns
1/2 teaspoon dried Thyme
1 bay leaf
4 cups (1 1/2 lb/750 g) diced plum (Roma) Tomatoes
2 cups (16 fl oz/500 ml) water
2 tablespoons unsalted Butter
1/4 lemon, sliced
1 teaspoon chopped fresh flat-leaf (Italian) Parsley, plus 4 sprigs
1/4 teaspoon file powder

1. Cover and refrigerate the bass fillets. Wash the reserved skeleton and head in cold water, then cut the skeleton into pieces and set aside.
2. To make the gumbo, in a large heavy saucepan over high heat, warm 4 tablespoons (2 fl oz/60 ml) of the vegetable oil. Add the onion, Flour and sausage and saute until the flour turns a medium-to-dark brown and exudes a slightly nutty aroma, 6-8 minutes. Add the bell Pepper, Celery, Garlic, 1 teaspoon Salt, peppercorns, Thyme, bay leaf, Tomatoes and water. Stir well. Add the reserved fish skeleton and head and bring to a boil. Reduce the heat to medium and skim away any scum that rises to the surface. Simmer, uncovered, for 1 hour.
3. During the final 10 minutes the gumbo is cooking, prepare the sea bass. Cut the fillets into 4 equal pieces and season lightly with salt. In a large frying pan over medium heat, melt the Butter with the remaining 1 tablespoon oil. Add the fish pieces and saute until lightly browned on the undersides, 4-5 minutes. Turn over the fish and cook until the fish is opaque throughout, 4-5 minutes longer, depending upon the thickness of the fillets.
4. Remove the gumbo from the heat. Remove and discard the skeleton and head. Strain through a fine-mesh sieve into a small saucepan, pressing hard on the solids to extract as much liquid as possible. Add the lemon slices and Parsley, then stir in the file powder.
5. To serve, ladle the gumbo into warmed soup bowls, place a piece of fish in each and garnish with the Parsley sprigs.