Arrange the spinach in a vegetable steamer; steam, covered, 3 minutes or until spinach wilts. Drain, squeeze dry, and coarsely chop. Combine spinach, mushrooms, and pesto in a medium bowl, stirring to combine; set aside.
Combine mozzarella, provolone, ricotta, and beaten egg in a medium bowl, stirring well to combine. Stir in 1/4 cup Parmesan, and set aside. Combine the pasta sauce and the tomato sauce in a medium bowl.
Spread 1 cup pasta sauce mixture in the bottom of a 6-quart oval electric slow cooker coated with cooking spray. Arrange 3 noodles over pasta sauce mixture; top with 1 cup cheese mixture and 1 cup spinach mixture. Repeat the layers, ending with spinach mixture. Arrange 3 noodles over spinach mixture; top with remaining 1 cup cheese mixture and 1 cup pasta sauce mixture. Place remaining 3 noodles over sauce mixture; spread remaining sauce mixture over noodles. Sprinkle with the remaining 1/2 cup Parmesan. Cover with lid; cook on LOW 5 hours or until done.