Tomato Sauce with Onion and Butter

"This is the simplest of all sauces to make, and none has a purer, more irresistibly sweet tomato taste." --Marcella Hazan

2 pounds fresh, ripe tomatoes, peeled and coarsely chopped, or 2 cups canned imported plum tomatoes, cut up, with their juice
5 Tbs. butter
1 medium onion, peeled and cut in half
Salt

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

Source of recipe: Marcella Hazan