Caramelize the Onions: Melt 2 tbsp butter in a skillet over medium heat. Sauté the 2 thinly sliced onions until golden brown, about 15 minutes. Place half of these onions into the bottom of the slow cooker.
Mix the Loaf: In a large bowl, gently mix 2 lbs ground beef, 1 cup breadcrumbs, 0.5 cup grated Parmesan cheese, 0.25 cup milk, 2 eggs, 1 tbsp Worcestershire sauce, 1 tsp salt, and 0.5 tsp black pepper until just combined.
Shape and Assemble: Form the beef mixture into a solid loaf shape and place it directly on top of the onions in the slow cooker. Top the meat with the remaining caramelized onions.
Add the Gravy Base: Pour the 10.5 oz can of French onion soup evenly over the meat and onions.
Cook: Cover the slow cooker and cook on LOW for 6 hours or on HIGH for 3 to 4 hours, ensuring the internal temperature reaches 160 F.
Melt the Cheese: During the final 15 minutes of cooking, sprinkle 1 cup shredded Swiss cheese evenly over the loaf. Cover to melt.
Garnish and Serve: Carefully remove the loaf, top with 1 tbsp chopped fresh parsley, and serve alongside the savory onion gravy and 4 bread rolls.