If making your own marinara sauce for this recipe you will want to start that first. Personally, I always make mine ahead of time and store it in the fridge or freezer. If using store bought marinara sauce you can skip this step.
Measure and combine ricotta cheese, 3 cups shredded mozzarella, spices and grated Parmesan cheese in a large bowl. Stir/mash together until incorporated. The mixture will be thick and slightly clumpy. Set aside.
Wash your mushrooms, spinach leaves, red bell pepper, and zucchini and dry. Use a sharp knife to chop into small pieces and add to a large prep bowl. I like my vegetables about ¼ inch large, but you can dice them even smaller if you want to to really meld into the sauce when cooking.
Once everything is prepped, start by spraying your crockpot with a little bit of cooking spray. This step helps the lasagna not stick and it come out clean when cooked. Next add a small amount of marinara sauce and spread it out completely coating the bottom of the slow cooker. Next, add a layer of dried lasagna noddles. It is okay to break some, overlap a bit to make them fit!Then add about ⅓ of the cheese mixture and spread it out evenly over the top of the noddles. Finally, add an even amount of each chopped vegetables and spread everything out into an even layer.
Repeat these steps until you have 3-4 layers of vegetarian lasagna and all of your ingredients are used. On the final layer I like to top with the remaining shredded mozzarella cheese.
Once layered, place the lid on the crockpot and cook the lasagna on LOW for 4 hours. After 4 hours, switch the crockpot OFF and let the lasagna chill out and rest in the slow cooker for 30 minutes to 1 hour. This rest time helps the lasagna soak up some of the sauce and make it let watery. If you skip this step the lasagna will still be delicious, but the sauce will be a bit runny. Serve topped with fresh parsley and enjoy!
Leftover Storage: Store leftover in an airtight container in the fridge for up to 4 days. Reheat lasagna in the microwave for 1-2 minutes or until warmed through.
Oven Option: Prepare the cheese mixture, and prep the vegetables according to the crockpot lasagna recipe instructions. In a deep 13x9 inch pan spay cooking spay then add a thin layer of marinara sauce. Continue to layer the lasagna according to the recipe instructions until you have used all of your ingredients.In a preheated oven (425 degrees F) cook the veggie lasagna on the center rack covered with foil for 45 minutes, then remove foil and bake an additional 10-15 minutes. Remove from oven and let rest for 15 minutes to allow lasagna to soak up some excess moisture.