In a large pot add some butter and once it starts to foam add the onions and sweat for a few minutes, then add the leek and sweat for few minutes more. Continue this process with the celery, herbs and the carrots. Add the tomato purée, mustard and cayenne pepper and cook for another few minutes whilst you keep stirring.
Put the crab shells in some fine muslin and smash with a rolling pin and add to the pot. Turn up the heat and add the brandy and allow the alcohol to burn off, then repeat with the white wine. Add the fish stock and bring to the boil, then turn down the heat to simmer for about one hour. Remove the muslin with the crab shells and make sure you drain them well into the pot.
Blitz really well using a blender and add the cream, pass through a fine strainer and season well with salt and pepper.