Add ground beef to a nonstick pan placed over medium heat, cooking until browned. Add the onion. Cook until beef is no longer pink and onion is softened.
Add other ingredients. Stir to combine well and cook over medium-low heat for 5 minutes. Remove from heat.
For each sandwich, butter one side of bread, top with cheese slice . Top with sloppy joe mixture. Place a second slice of cheese if you like. Place another piece of bread with butter side up.
Heat griddle or nonstick pan over medium heat. Cook until bottom is golden, lift sandwich with a spatula, flip sandwich over. Cook until cheese is melted.