1Spray 5- to 6-quart slow cooker with cooking spray. Spread 3/4 cup of the tomato pasta sauce in bottom of slow cooker.
2In large bowl, mix bell peppers, onion and spinach; stir in remaining tomato pasta sauce and the tomato sauce.
3Layer 3 lasagna noodles, broken into pieces to fit, over sauce in slow cooker. Top with one-third of the Alfredo sauce (about 1/2 cup), spreading to cover noodles completely. Top with 5 of the cheese slices, overlapping if necessary. Top with one-third of the vegetable mixture (about 2 cups), spreading evenly. Repeat layers twice. Sprinkle Parmesan cheese over top.
4Cover; cook on Low heat setting 5 to 6 hours.