1. Puree the tomato sauce, spinach, salt and pepper in a blender. Add 1/2 cup basil, and pulse to combine. Generously coat a slow cooker insert with cooking spray. Spread 1/2 cup sauce in the bottom of the slow cooker. Layer in three lasagna noodles, breaking to fit, trying not to overlap too much. Dollop about 1/2 of the ricotta over the noodles. Top with 2/3 cup sauce and 2/3 cup mozzarella cheese. Repeat with another layer of noodles, the remaining ricotta, 2/3 cup sauce and 2/3 cup mozzarella cheese. Top with the remaining noodles and 2/3 cup sauce. Reserve the remaining sauce (about 3/4 cup) and 2/3 cup mozzarella cheese.
2. Cover slow cooker, and cook for three hours on the low setting. Unplug the slow cooker.
3. Heat the remaining 3/4 cup sauce in a small saucepan or in a microwave until steaming. Remove the lid of slow cooker. Pour the sauce over the lasagna, and top with the remaining 2/3 cup cheese. Cover and let sit for 10-15 minutes before serving. Scatter chopped basil on top for garnish.