To save ourselves a bit of work, I recommend chopping the tomatoes in a food processor or blender before cooking them. A few pulses will make a chunky sauce, and longer processing will make a very smooth sauce. Conversely, if you like a very chunky sauce, skip this step altogether and let the tomatoes break down naturally as they simmer. You can also chop the tomatoes by hand, run them through a food mill, or purée them with a stick blender after they've been cooking.