Put either the prepared fresh tomatoes or the canned in a saucepan, add the butter, onion and salt, and cook uncovered at a very slow but steady simmer for about 45 minutes, or until it is thickened to your liking and the fat floats free from the tomato. Stir from time to time, mashing up any large pieces of tomato with the back of a wooden spoon. Taste and correct for salt. Discard the onion before tossing the sauce with pasta.

Making Fresh Tomatoes Ready for Sauce:

Unless the recipe indicates otherwise, fresh ripe tomatoes must be prepared to use for sauce following one of the two methods given below. The blanching method can lead to a meatier, more rustic consistency. The food mill method produces a silkier, smoother sauce.

The blanching method: Plunge the tomatoes in boiling water for a minute or less. Drain them and, as soon as they are cool enough to handle, skin them and cut them into coarse pieces. The food mill method: Wash the tomatoes in cold water, cut them lengthwise in half, and put them in a covered saucepan. Turn on the heat to medium and cook for 10 minutes. Set a food mill fitted with the disk with the largest holes over a bowl. Transfer the tomatoes with any of their juices to the mill and purée.