Skim off the protein scum that rises to the surface. Cover and reduce heat to low. Simmer 30 minutes. Add carrots, cover, and cook another 30 minutes. Remove onions and bones and discard.
Force tomatoes through a sieve and add to the pot with the vegetables.
Cover and cook for 30 minutes. Add lemon juice and sugar to taste. Cook five minutes. Remove meat from the pot and mince. Return to the pot and serve very hot with a dollop of sour cream in each bowl for garnish.