Heat oil in a very large skillet or Dutch oven over medium-high heat. Add onions & mushrooms, and sauté for about 7 minutes, until mushrooms release their juices. Add garlic and spinach and stir for 30 seconds.
Add in pasta sauce, water, and salt. Bring to a boil, and reduce heat to a simmer. Simmer for 5 minutes, stirring occasionally.
Meanwhile, stir together ricotta, egg, 1 cup mozzarella, 2 tablespoon Parmesan, parsley, and Italian seasoning. Set aside.
Spray a slow-cooker base with non-stick spray. Spoon about ⅓ of the veggie-sauce mixture into slow-cooker. Top with 2 lasagna noodles {you might have to break them a little to get them to fit}. Spoon ½ ricotta mixture on top of lasagna noodles.
Put ⅓ of veggie-sauce mixture over ricotta. Top with two more lasagna noodles, then top with other ½ of ricotta mixture. Pour any remaining sauce over ricotta.
Cover and cook on Low for 3.5 to 4.5 hours until lasagna noodles are tender. {Mine took 3 hours 45 minutes}. Be careful when taking to top off the slow-cooker so you don't let any condensation get into the lasagna.
Sprinkle with remaining ½ cup mozzarella and 2 tablespoon Parmesan. Put top back on and let cheese melt for about 10 minutes.
Cut into 6 pieces and enjoy!
Notes
*I recommend turning this to Warm after 4 hours of cooking, so it doesn't dry out.