Sweat in butter and oil for 8 to 10 minutes the onions and carrots cut in brunoise, without browning.

Add the crushed and brushed shells. Let sweat, frequently stirring. In this case I preffered langoustines shells.

Add the brandy and flambé. The alcohol must be warm to ignite easily.

Deglaze with white wine and reduce for a few minutes. Add the seeded and crushed tomatoes, the tomato paste, tarragon and crushed garlic.

Moisten with boiling fish broth and add the bouquet garni.

Bring to a boiling point and simmer over medium heat for 30 minutes.

When finished, filter through a chinois or fine sieve.

Bring to a boiling point and bind with cornstarch or beurremanie. Dissolve a bit of cornstarch in cold broth and pour in the boiling sauce, whisking. Pour the cornstarch gradually until it reaches the expected thickness, or whisk in the butter until thick enough. Filter again through a chinois and whisk the butter in. Keep warm in a bain Marie.