I adapted this recipe from My Favorites. I cut the recipe in half and substituted plain tomato sauce for the tomato soup. (By the way, canned tomato soup is usually MSPI-friendly, I just prefer using tomato sauce.)
Stir in the onions, tomato sauce, ketchup, vinegar, brown sugar and Worcestershire sauce into the beef mixture. Season with salt and garlic powder. Heat to a simmer over low heat, and cook until thoroughly heated and onions are tender, stirring frequently to prevent it from burning on the bottom. Adjust seasoning to taste.
Spoon the hot beef mixture onto buns and serve.