In the bowl of a food processor, combine the bacon, onion, and garlic. Process the bacon is entirely ground and no large chunks remain.
Preheat the broiler. Transfer the bacon mixture to a large bowl and mix in the ground beef, breadcrumbs, parsley, egg, and seasonings. Roll the meat mixture into golf ball-sized balls and place on a baking sheet.
Broil the meatballs until browned on top, about 7 minutes. (Browning times will vary.)
Meanwhile, whisk together the sauce ingredients in the slow cooker. Add the meatballs and cook on high for 1-2 hours or low for 3-4 hours. Switch the slow cooker to warm and serve.