Pour half of the marinara sauce in a 6-quart or larger slow cooker; set aside.

Place the eggs in a large bowl and whisk with a fork to break them up. Add the panko, Parmesan, onion, parsley or basil, garlic, salt, oregano, and pepper and stir to evenly moisten and combine. Add the ground chicken and, using clean hands, mix everything until combined (don’t squeeze or overwork the mixture).

Fill a medium bowl with water. Form the meat mixture into 1 ½-inch meatballs (2 tablespoons each), wetting your hands in the bowl of water as necessary to keep the mixture from sticking. As each meatball is formed, place in the sauce (you will have about 1 ½ layers of meatballs). Pour the remaining marinara sauce evenly over the meatballs.

Cover and cook on the LOW setting until the meatballs are cooked through and the flavors meld, 6 to 8 hours. Sprinkle with the mozzarella cheese and cook covered in the last 15 minutes if desired. Sprinkle with more parsley or basil and serve with more Parmesan.