In a skillet over medium heat, melt butter and sauté onions until deeply caramelized, approximately 15 minutes.
In a large bowl, combine ground beef, soup mix, breadcrumbs, eggs, half of the beef broth, and Worcestershire sauce.
Form the beef mixture into an oval loaf shape.
Place the loaf into the slow cooker and top with the caramelized onions.
Pour the remaining beef broth around the base of the loaf in the slow cooker.
Cover and cook on LOW for 6 hours or until the internal temperature reaches 160 degrees Fahrenheit.
Top the loaf with shredded Gruyere cheese during the final 15 minutes of cooking.
Rest the meatloaf for 10 minutes before slicing and serving.