In a large bowl mix together wheat bread and milk until it forms a paste. Add ground turkey, spinach, feta, onion, Worcestershire sauce, salt, pepper, oregano, garlic, onion powder, and breadcrumbs.
Mix well until all of the ingredients are well incorporated into the meat. From the mixture into 18-20 meatballs and place into the slow cooker. Pour 3 cup of marinara sauce over the meatballs. Cover and cook on low for 4-5 hours.