TOMATO SAUCE WITH ONION AND BUTTER   1 (28 oz.) can peeled San Marzano (or another) brand tomatoes, crushed with the juice 1/2 c (1 stick) butter 1 large onion, peeled and cut in half salt to taste heavy cream (optional) 1 1/2 lbs. pasta, cooked according to package directions grated parmesan cheese   Put all ingredients in saucepan and cook, uncovered, at a slow but steady simmer, 45 minutes or until fat floats free from tomatoes. Stir from time to time, mashing any large pieces of tomato in pan with back of wooden spoon. Taste and correct for salt. Add a little cream if desired. Discard onion before tossing sauce with pasta.