In a medium mixing bowl pour milk over the bread crumbs and let sit for 5 minutes.
Add egg yolks, parmesan cheese, salt, and pepper and stir to make a paste.
In a large bowl combine beef, pork, and onion.
Add the breadcrumb mixture to the meat mixture. Use your hands to make sure it is evenly combined.
Shape the mixture into meatballs, about 1 to 1.5 inches in diameter.
In a hot skillet brown each side of the meatball. You don’t need to cook it thru. You just want to get some nice caramelization on each side. Don’t skip this step
Pour half of the marinara sauce into the bottom of the slow cooker.
Arrange the meatballs in a single layer in the slow cooker.
Pour the remaining marinara sauce over the meatballs, ensuring they are fully covered.
Cover and cook on low for 6 to 8 hours, or on high for 3 to 4 hours, until the meatballs are cooked through.
Once cooked, garnish with fresh chopped parsley, if desired, and serve hot.