Peel onion and cut into small cubes. Drain tomatoes, reserving juice and cut into small pieces. Heat oil and sauté onions until translucent. Stir in tomato paste and continue to sauté. Add tomato pieces and sauté briefly. Add broth and reserved tomato juice. Bring to a boil, reduce heat, and simmer for 20 minutes. Season with salt, pepper, and sugar to taste.
Meanwhile, peel garlic and finely mince. Cut the bread into cubes. Melt butter in a pan. Add bread, herbs, and garlic. Sauté until the bread toasts to a golden brown. Lightly salt and remove from the pan. Puree soup with the blender and return to a simmer. To serve, ladle the soup into warmed bowls and sprinkle with croutons. Top with minced rosemary if desired.