Layer 3 lasagna noodles, broken into pieces to fit, over sauce in slow cooker. Spread half of the ricotta cheese over noodles; sprinkle with 1/4 cup of the mozzarella cheese and half of the spinach. Top with one-third of the tomato sauce mixture (about 1 1/2 cups). Repeat layering of noodles, cheeses and spinach. Top with remaining 3 noodles and sauce. Save remaining 1 cup mozzarella cheese in refrigerator.