Prepare the slow cooker by lining it with a large piece of foil folded into a strip (sling) to assist with removal later. Lightly grease the foil.

In a large skillet over medium heat, melt the butter. Add the sliced onions and cook slowly, stirring occasionally, for about 20 minutes until they are golden brown and soft.

Pour the beef broth into the skillet with the onions, stirring to scrape up any browned bits (fond) from the pan. Simmer for 2 minutes, then remove from heat.

In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, milk, eggs, Worcestershire sauce, salt, and pepper. Mix gently with your hands just until combined; do not overmix.

Shape the beef mixture into a firm loaf and place it into the prepared slow cooker on top of the foil sling.

Spoon the caramelized onion and broth mixture evenly over the top of the beefloaf.

Cover and cook on low for 6 to 8 hours, or until the internal temperature reaches 160°F (71°C).

About 10 minutes before serving, arrange the Swiss cheese slices over the hot beefloaf. Cover the crockpot again and let the cheese melt.

Using the foil sling, carefully lift the beefloaf out of the slow cooker. Slice and serve warm, ensuring each piece has onions and cheese.