Drop the tomatoes into boiling water, a few at a time. Let the water return to a boil, then remove the tomatoes and drain. Peel and chop.
In a large saucepan, cook the onion, celery, and carrots in the olive oil, covered, over medium-low heat for about 15 minutes, stirring a few times. Uncover and stir, over the heat, for 5 to 10 minutes longer, or until the ve