Mix the meat mixture: In a large bowl, combine 2 lbs ground beef, 1 cup breadcrumbs, ½ cup grated Parmesan, ¼ cup milk, 2 eggs, 1 tbsp Worcestershire sauce, 1 tsp salt, and ½ tsp pepper. Mix until well incorporated but be careful not to overwork the meat.
Shape the meatloaf: Form the mixture into a loaf shape that fits your crockpot.
Caramelize the onions: In a skillet, melt 2 tbsp butter over medium heat. Add 2 large sliced onions and cook until golden brown, about 15-20 minutes, stirring occasionally. The onions should be soft and caramelized.
Add broth: Once the onions have caramelized, pour in 1 cup beef broth and stir well. Allow the mixture to simmer for an additional 5 minutes.
Layer the ingredients: Pour half of the caramelized onion mixture into the bottom of the crockpot. Place the shaped meatloaf on top and spread the remaining onion mixture over it.
Slow cook the meatloaf: Cover and cook on low for about 6-8 hours or until the beef is fully cooked and reaches an internal temperature of 165°F.
Add Swiss cheese: About 10 minutes before serving, lay 8 slices Swiss cheese over the meatloaf, allowing it to melt.