To a skillet over medium high heat add your onions and butter and let caramelize about 10 minutes until lightly browned. Add to the slow cooker.
To a large bowl add your ground beef, beefy onion soup mix, bread crumbs, eggs, milk and worcestershire sauce.
Dice up 5 slices of the swiss cheese and add that to the mix.
Mix the meatloaf mixture until combined and shape into a loaf.
Add the loaf to the slow cooker.
Pour the french onion soup over the top.
Cover and cook on low for 4 hours or high for 3 hours.
Five minutes before serving top with the remaining swiss cheese slices and cover until melted.
Garnish with parsley.
Slice and serve.