Rinse and chop the tomatoes. Peel and finely chop the onions and garlic. Melt the butter in a saucepan, and shortly sauté the onions, garlic, and tomatoes. Add the rosemary, bay leaves, and thyme, then add the broth. Cover, and simmer for 20-30 minutes over low heat.
Strain the soup through a fine strainer, reheat, and season to taste with sugar, salt, and pepper. Divide into bowls, and garnish with the basil leaves.