Crush the shellfish or crabs in a liquidiser or food processor. Sauté them in the hot oil in a saucepan for 5 minutes, stirring all the time. Deglaze with the cognac, add the white wine and then the fish fumet, and salt. Stir in the tomato purée and simmer for 20 minutes.
Strain the stock through a conical strainer, pressing the shells firmly with a spoon or th