The longer cooking time allows the flavor of tomatoes to sweeten and become more complex. Because it's mostly unattended, you can cook pasta and prepare a salad while it does its thing. Adapted from Marcella Hazan. Essentials of Classic Italian Cooking.
Heat a medium saucepan over medium heat. Add the olive oil, followed by the onions and cook until soft and beginning to brown, 8-10 minutes.
Add the carrots and celery to the pan, and cook for another 2-3 minutes.
Add the tomatoes with their juices to the pan, using a wooden spoon to crush them and break them up. Add a generous pinch of salt and simmer for 45 minutes up to one hour.
Adjust the seasoning to taste. Serve with spaghetti or penne.