4 medium zucchini, sliced ⅛” thick. (UPDATE: Since some readers are saying that this lasagna become a little soupy I made this lasagna again but this time I grilled the zucchini slices before I put on the slow cooker. It helped to draw out some moisture!
In a medium bowl mix ricotta cheese, egg, Parmesan cheese, spinach. Stir well and set it aside.
Lightly coat the inside of the slow cooker with cooking spray.
Ladle about 1 cup of homemade tomato sauce on the bottom of the slow cooker and spread it well.
Place some of the ricotta cheese mixture and top with the mozzarella cheese.
Repeat the layers until all your ingredients are all used up.
Top the lasagna with mozzarella and Parmesan cheese.
Cover and cook on high for 3-1/2 to 4 hours.
Turn off the slow cooker and let stand for about 1 hours. (UPDATE: Since some readers said the lasagna became a little soupy, I made it again and I let it stand for about 3 HOURS before serving. I found it was great because the liquid absorbed much more – Please, do not forget this step).