Add the shallots, leek, carrot, fennel, garlic and celery and fry for 10 mins over a low heat until soft. Add the tomato puree, chopped tomatoes, white wine and brandy and reduce by half. Add the chopped salmon and cod and cook for 5 mins. Add the stock, bay leaf, tarragon and cayenne pepper, cover and simmer for 40 mins. Blitz the mixture and then pass through a sieve. Really push the mix through the sieve to get as much flavour as possible.
For the stew
Pour the sauce back into the casserole dish and bring up to the boil.
Add the fish and vegetables and simmer for 3 to 4 minutes or until the fish is cooked through. When cooked stir in the double cream and mix through the parsley.
Stir in the lemon juice and season with salt and pepper.