This tomato sauce recipe is from Marcella Hazan's "Essentials of Classic Italian Cooking" and is used to make her deliciousTortelloni Stuffed with Swiss Chard, Prosciutto, and Ricotta.
Place tomatoes in the bowl of a food processor and pulse twice to break up tomatoes; set aside.
Cook butter and onion in a medium skillet over medium heat until browned. Add carrot and celery; cook for 2 minutes. Add tomatoes, and reduce heat until mixture just barely simmers. Cook, stirring occasionally, for 30 minutes. Season with salt.
Add heavy cream and return saucepan to medium-high heat. Cook, stirring, until sauce is heated through, about 1 minute. Use immediately.