In a large bowl, combine ground beef, breadcrumbs, egg, salt, pepper, and garlic powder; form into 1.5-inch meatballs.
In a large skillet over medium-high heat, melt butter and sear meatballs until browned on all sides, then transfer to the slow cooker.
In the same skillet, add sliced onions and cook for 5 minutes until slightly softened.
Whisk together beef broth, dry French onion soup mix, and Worcestershire sauce in a small bowl.
Place the onions over the meatballs in the slow cooker and pour the broth mixture over the ingredients.
Cover and cook on LOW for 4 hours until meatballs are cooked through and onions are tender.
Top the meatballs with Provolone slices and shredded Gruyère, then cover for 10 minutes until cheese is fully melted and bubbly.
Garnish with fresh parsley before serving directly from the slow cooker.