Apply cooking spray to the inside of a 4-quart slow cooker. Put oil into a big pan and set it over medium-high heat. Sauté the mushrooms for 3 minutes, tossing them around once. Stir in the spinach and continue cooking for another three minutes, or until the mushrooms are soft and the spinach has wilted. Add salt, Italian seasoning, tomato sauce, undrained tomatoes, and stir. Heat till boiling. Put the pot on low heat and simmer for three minutes, stirring once or twice. In a separate dish, mix together the ricotta cheese, 1/4 cup of Parmesan, and pepper; put aside.
2. Cover the base of the slow cooker with 3/4 cup of the sauce mixture. Arrange three broken lasagna noodles on top of the sauce mixture. Vary the amount of sauce, ricotta cheese, and Italian cheese that you use. Sprinkle 3/4 cup of each on top. After that, scatter 3/4 cup of the sauce mixture over top. Then, break the remaining 3 lasagna noodles to fit. Dispense the leftover sauce mixture over the noodles.
Turn heat to low and simmer, covered, for 5 hours, or until noodles are soft. Mix in the remaining half cup of Italian cheese and quarter cup of Parmesan cheese. Remove lid and let aside for 5 minutes to allow cheese to melt.
