In a saucepan over medium-low heat, melt butter. Cook chopped onion in butter until tender; add tomato sauce, sweet pickle, vinegar, parsley flakes and sugar. Simmer, uncovered, for 10 minutes.
To Freeze: Freeze in a freezer safe container with sauce separated.
To Reheat: Arrange bacon slices over pork tenderloin. Place tenderloin on rack in a shallow roasting pan. Tuck thin end of meat under. Roast, uncovered, at 350° for about 25 to 35 minutes, or until internal temperature is around 160°.Serve sauce with roasted pork tenderloin.