In a medium bowl mix ricotta, egg, Parmesan cheese, spinach, parsley, and salt & pepper. Stir well and set it aside.
Lightly coat the inside of the slow cooker with cooking spray. Ladle about 1 cup of tomato sauce on the bottom of the slow cooker and spread it around to coat.
Place a single layer of noodles on top of the sauce, breaking some if needed to fit in the slow cooker. Layer about a cup of the ricotta mixture on top of the noodles, then top with ⅓ of the cheese, sauce, then zucchini slices.
Repeat the layers until all your ingredients are all used up, ending with sauce and an extra sprinkling of Parmesan cheese.
Cover and cook on HIGH for 2-½ to 3 hours. Turn the slow cooker to WARM and let stand for 30 minutes. This will allow the liquid to be absorbed. Cut with a knife and spoon onto plates. Sprinkle with fresh parsley or basil. Serves 4-6 people.
Note: You can make this dish low-carb by omitting the lasagna noodles.