My mother found this recipe about 40 years ago when recipes used to be printed on the back of matchbooks. This is a family favorite. Our daughter now makes it for her family and her 3 kids (all under 6)love this. My husband and I love it with either a baked potato, squash, or au gratin potatoes and a salad.
Lightly mix ground beef, crumbs, onion, 1/2 cup tomato sauce, egg and seasonings. Shape into loaf in shallow pan. Combine reserved tomato sauce with the brown sugar, mustard and vinegar. Pour over loaf. Bake at 350 for 1-1/4 hour.