In a large bowl, combine the turkey, dried oregano, ground fennel, garlic, spinach, breadcrumbs, Parmesan cheese and egg. Sprinkle with the kosher salt and freshly ground pepper to taste. Using a fork, gently stir to incorporate (don’t work the mixture too much, otherwise the meatballs could turn tough). Gently form 16 golf ball-sized balls (they’ll be sticky), and nestle them in the slow cooker in a single layer.