To a large bowl, add ground beef, ground pork, salt, pepper, onion, garlic, and parsley. Stir to combine.
To a separate medium bowl, add bread crumbs, milk, and egg. Stir to combine.
Add the milk mixture to the meat mixture and combine with your hands (without overworking the meat).
Form the mixture into 1-inch balls and set aside.
To a Dutch oven over medium heat, add butter.
Once the butter has melted, add the meatballs and cook through, rotating to brown all sides, about 10-12 minutes. (Work in batches to not overcrowd the pot.)
Transfer the cooked meatballs to a plate. Tent to keep warm as you make the soup.