Combine egg, ricotta, spinach, mushrooms and zucchini in a large bowl.
Combine crushed and diced tomatoes and their juice, garlic and crushed red pepper (if using) in a medium bowl.
Generously coat a 6-quart or larger slow cooker with cooking spray. Spread 1 1/2 cups of the tomato mixture in the slow cooker. Arrange 5 noodles over the sauce, overlapping them slightly and breaking into pieces to cover as much of the sauce as possible. Spread half of the ricotta-vegetable mixture over the noodles and firmly pat down, then spoon on 1 1/2 cups sauce and sprinkle with 1 cup mozzarella. Repeat the layering one more time, starting with noodles. Top with a third layer of noodles. Evenly spread the remaining tomato sauce over the noodles. Set aside the remaining 1 cup mozzarella in the refrigerator.
Put the lid on the slow cooker and cook on High for 2 hours or on Low for 4 hours. Turn off the slow cooker, sprinkle the reserved mozzarella on the lasagna, cover and let stand for 10 minutes to melt the cheese.
Equipment: 6-quart (or larger) slow cooker
Tip: The dark gills found on the underside of a portobello mushroom cap are edible, but can turn a dish an unappealing gray color. If you like, gently scrape the gills off with a spoon.
For easy cleanup, try a slow-cooker liner. These heat-resistant, disposable liners fit neatly inside the insert and help prevent food from sticking to the bottom and sides of your slow cooker.