Spray crockery pot of slow cooker with nonstick cooking spray; set aside. In a small bowl, mix together mozzarella, ricotta, Parmesan, egg, oregano, and garlic powder.
Spread 2 tablespoons of pasta sauce in bottom of pot. Sprinkle 1/2 of zucchini over sauce and top with 1/3 of the cheese mixture. Break 2 noodles into pieces to cover cheese. Spread 2 tablespoons of sauce and then layer 1/2 the spinach and 1/2 the mushrooms. Repeat layering, ending with cheese and the remaining sauce. Firmly press ingredients into pot.
Cover and cook over low heat for 4–5 hours. Allow lasagna to rest for 20 minutes before cutting into wedges to serve. Spoon a little extra sauce over each serving and top with a basil leaf, if desired.