Pour half of the marinara sauce into a 6-quart slow cooker, spreading it evenly across the bottom. Set aside the remaining sauce for later.

In a large bowl, whisk the large eggs until well beaten. Combine them with panko breadcrumbs, Parmesan cheese, minced onion, parsley or basil, garlic, kosher salt, and oregano.

Gently fold the ground chicken into the egg and breadcrumb mixture until just combined.

With damp hands, form the mixture into 1½-inch meatballs and place each one in the slow cooker over the marinara sauce.

Carefully pour the remaining marinara sauce over the meatballs, ensuring they’re completely covered.

Cover the slow cooker and cook on LOW for 6-8 hours, until the meatballs reach an internal temperature of 165°F.

If including mozzarella, add it on top of the meatballs during the last 15 minutes of cooking.

Once cooked, sprinkle additional parsley or basil on top and serve.