Mix the ground turkey, 1/4 cup spaghetti sauce, bread crumbs, onion, eggs, Parmesan cheese in medium bowl.
Put half of the turkey mixture into a loaf baking pan. Create a 'trench' as you will be adding the spinach to it. Leave about 1" trench in the center of mixture and about 1" on all sides.
Mix the spinach and mozzarella cheese together then transfer and fill the trench. Fill the trench with the remaining meat mixture.
Bake about 45-50 minutes in a 350 degree F preheated oven. Then spoon the remaining spaghetti sauce over meat loaf. Bake the meatloaf for about 15 minutes more. Let stand for 10 minutes.