In a mixing bowl, combine ground beef with breadcrumbs, Parmesan, egg, milk, and spices. Mix gently without overworking to keep the meatballs tender.

Shape the mixture into uniform meatballs about one inch in diameter. Be gentle to ensure they remain soft.

Lightly spray the inside of your slow cooker with cooking spray. Place the meatballs in a single layer.

Melt butter in a pan and add the sliced onions. Cook them until they turn a deep golden-brown, enhancing the flavor.

Pour the caramelized onions and beef broth over the meatballs, making sure they are fully covered.

Cover the slow cooker and set it on low heat. Let the meatballs cook for several hours until they are tender and infused with the sauce's flavors.

In the last 30 minutes of cooking, sprinkle extra Parmesan cheese on top for a delicious melted crust.

Carefully transfer the meatballs to a platter and drizzle the remaining onion sauce over them for a beautiful presentation.