In a large mixing bowl, combine the ground beef (or turkey), breadcrumbs, grated Parmesan, chopped parsley, egg, minced garlic, chopped onion, salt, and black pepper. Mix well until all the ingredients are fully incorporated.

Shape the mixture into meatballs, about 1 to 1.5 inches in diameter. You should have approximately 24 meatballs.

In a skillet, heat the olive oil over medium heat. Brown the meatballs for about 3-4 minutes on each side until they're golden. This step adds extra flavor but can be skipped for a quicker prep.

Place the meatballs directly into the slow cooker. Pour the marinara sauce over the top, making sure all meatballs are coated.

Cover the slow cooker and cook on low for 6 hours or on high for 3 hours, until the meatballs are cooked through and tender.

Serve hot over pasta, in sub rolls, or on their own with a sprinkle of extra parsley and cheese if desired.

For a spicy kick, consider adding red pepper flakes to the sauce.

You can make the meatballs ahead of time and freeze them before cooking. Simply thaw and place in the slow cooker with sauce when ready to serve.

Leftover meatballs can be stored in an airtight container in the refrigerator for up to 3 days or frozen for longer storage.