In a large bowl, add the onion, garlic, egg, bread crumbs, ketchup, parsley, Parmesan, Pecorino, salt and pepper.
Using your hands, mix in the turkey meat. (NOTE: I sautéed the onion and added it to the sauce rather than adding it to the meatball mixture.)
Pour the entire jar of Marinara Sauce into the bottom of your slow cooker.
Shape the meat mixture into 1 1/4-inch-diameter ball. Gently place the meatballs in the sauce. You do NOT have to brown them first. (That’s what I LOVE about slow cooking them. You can skip the browning part!)
Cover and cook on LOW for 4-6 hours. Serve over your choice of pasta.