Cut up the shells as much as possible with a heavy knife. Heat the butter in a large saucepan. Add the onion, carrot, celery and shells and cook for about 5 minutes until the vegetables are softened, pressing down on the shells as much as you can. Stir in the tomato purée followed by the brandy. Cook until the brandy has evaporated, then add the wine and cook for 2 minutes until reduced.
Add the tomatoes, stock, bay leaf and rice. Bring to the boil, then simmer for about 15 minutes.
Pour the mixture into a blender (discarding any very large shells) and whizz until the shells have broken up and are no larger than your fingernail. Pass through a fine sieve, pressing out as much juice as you can.
Stir in the cream, season, scatter over the cooked shellfish and heat through. Ladle into bowls, and garnish with finely chopped tarragon.