Clean the crab and remove the meat from the shell.
Reserve the meat. Use the shell to make the crab stock.
Heat the olive oil in a heavy stockpot.
Add the crab shells, carrot, onion and celery; brown lightly.
Add brandy and ignite. When flames have expired, add the water, garlic, bay leaf, tarragon and tomato paste.
Bring stock to a low simmer for 2 to 3 hours and reduce to 1 quart.
Strain through a fine sieve and refrigerate until ready to use.
Make a roux in a heavy saucepan by melting 4 ounces (1 stick) butter over medium heat. When foam subsides, add the flour all at once. Stir constantly, reduce the heat to low and cook, stirring until it is blond-colored (about 5 minutes).
Add crab stock, cup by cup, whisking thoroughly after each addition. Bring to a boil, reduce to a simmer for 20 minutes, skim frequently.
Add heavy cream, juice from the lemon, tabasco, worcestershire, old bay and white pepper. Bring back to simmer for 10 minutes.
Add the whiskey, sherry, 2 tablespoons butter and the reserved crab meat; cook 1 minute to heat through. Salt to taste.