Combine bell pepper, mushrooms, and spinach in a medium bowl. Combine pasta sauce, tomatoes, ricotta, and ½ cup Parmesan in a large bowl.
Spread 1 cup of the sauce mixture in a 6-qt. slow cooker. Place 3 noodles over the sauce, breaking them as needed to fit in one layer. Spread another 1 cup sauce over the pasta. Layer on one-third of the vegetable mixture, 3 more noodles, and 1 cup sauce. Sprinkle with ½ cup mozzarella. Continue layering one-third of the vegetables, 1 cup sauce, ½ cup mozzarella, and the remaining noodles. Top with a final layer of the remaining vegetables, the remaining sauce, and the remaining 1 cup mozzarella.
Cover and cook on Low for 5 hours.
Turn off and unplug the slow cooker. Let the lasagna stand, covered, for 1 hour (see Tip). Serve topped with the remaining ¼ cup Parmesan and basil.
Tip: To prevent condensation building up on the underside of the lid during the 1-hour rest time, place a paper towel under the lid during the rest time.