In a medium mixing bowl, combine the breadcrumbs, milk, and egg. Let the mixture stand for 3 minutes or until the crumbs have softened. Add the onion, parsley, garlic, salt, and pepper, mix in. Using your hands, gently mix in the ground beef until well combined.
For dinner size meatballs, use a standard size cookie scoop to form meatballs. For appetizer size meatballs, use two teaspoons.
If your slow cooker does not have a saute function, in a large nonstick skillet set over medium heat, add the olive oil. Once hot, add the meatballs and brown on all sides, about 10 minutes. Transfer the meatballs to the slow cooker.
Whisk together the ingredients for the sauce in a small mixing bowl, then pour over the meatballs.
Turn slow cooker on low for 3-4 hours. Serve meatballs warm.