Peel and wash the carrot and celery and cut into small cubes. Heat the oil in a pot. Add the carrot and celery and fry until transparent. Add fish stock and strained tomatoes. Bring to the boil and simmer for 5 minutes.
Meanwhile peel the onion, cut it into rings and add it to the hot soup. Add crabs and stir carefully. Season with lemon juice and coarse pepper. Chill the soup. Garnish with parsley.
It goes well with toast.