Wash lobster well.
Separate the head from the body/tail by inserting knife into first membrane layer holding the two together.
Combine head, celery, carrot, onion with half the butter in large sauce pan saute everything for few minutes, add the flour little by little, the tomato paste, the water, salt & pepper, parsley, and bay leaf.
Bring to a boil; simmer 15 minutes, skimming as necessary. Strain, reserve 8 cups stock and tail.
Using scissors, extract meat from reserved tail by cutting through shell and carefully peeling away meat. Slice in medallions.
Melt the other half of butter in large saucepan over medium heat. Add lobster medallions and cook for 10 seconds do not over cook the lobster; add reserved stock and heavy cream.
Bring to boil, stirring constantly until thick and creamy. Finish with Sherry wine, cognac, salt and pepper to taste.