Heat olive oil in a casserole dish and fry the shallots, leek, carrot, fennel, garlic, and celery for 10 minutes until soft.
Add tomato purée, chopped tomatoes, white wine, and brandy and simmer until the liquid is reduced by half.
Stir in 300g of chopped salmon and cod and cook for 5 minutes.
Add 900ml of fish stock, bay leaf, tarragon, and cayenne pepper, then cover and simmer for 40 minutes.
Blend the mixture and pass it through a sieve, pushing it through to extract maximum flavor.
Return the sieved sauce to the dish, bring it to a boil, and add the assorted seafood and cooked vegetables.
Simmer for 4 minutes until the fish is cooked, then stir in the double cream and parsley.
Add lemon juice, season with salt and pepper, and serve with bread and mayonnaise.