Coat slow cooker bowl with nonstick cooking spray. Stir together onion, garlic, turkey, oregano, salt and pepper; set aside.
In a small bowl, stir together ricotta, 1 cup of the shredded cheese and the spinach.
In slow cooker bowl, layer about 8 of the uncooked noodle halves, overlapping as necessary. Spread half of both the meat mixture and the ricotta mixture over noodles, then top with about 1 cup of the tomato sauce and 1/4 cup water. Continue layering with remaining noodles, meat, ricotta mixture, 2 cups of the sauce and an additional 1/4 cup water. Top layers with 8 noodle halves and remaining 1/2 cup tomato sauce.
Cover and cook on HIGH for 4 hours or LOW for 5 1/2 hours, or until internal temperature registers 160° F on an instant-read thermometer. Sprinkle remaining 1 cup shredded cheese on top for last 30 minutes of cooking time or until melted.