Peel onions and carrots. Dice onions. Wash and slice the carrots. Heat the fat. Brown onions and carrots in it. Sweat tomato paste briefly. Season with salt, cayenne pepper and bay leaf. Add 200 ml water, bring to the boil, cover and cook for about 15 minutes.
Wash and quarter the tomatoes. Add tomatoes to vegetables and simmer covered for another 30 minutes. Remove the bay leaf. Pass the soup through a fine sieve. Season with salt, pepper, cognac and lemon juice
Whip the cream briefly. Rinse and drain the crabs if necessary. Serve the soup with a dollop of cream in deep plates. Garnish with prawns, pepper and dill