Heat the oil in a large saucepan over medium high heat. Add the onions; sauté for 2 minutes. Add the garlic; sauté for 30 seconds. Add the celery and carrots; saute for 8-10 minutes or until vegetables are tender. Add the 6 cups of tomato; cook 2 minutes. Reduce heat to low and stir in chicken stock. Simmer, covered for 30 minutes.
Uncover; simmer for an additional 45 minutes or until reduced to about 6 cups. Drain broth mixture through a fine sieve into a large bowl; discard the solids. Return the broth to the pan. Stir in the remaining tomatoes and season to taste with salt and freshly ground black pepper. Bring to a boil. Reduce heat and simmer, uncovered for 10 minutes.
Remove the soup from the heat. Stir in the basil, lemon zest and butter. Adjust seasonings to taste with salt and freshly ground black pepper.