1. Spray inside of 4-quart slow cooker with cooking spray. Heat oil in large skillet over medium-high heat. Add mushrooms; cook 3 minutes, stirring occasionally. Add spinach; cook 3 minutes more or until mushrooms are tender and spinach is wilted, stirring occasionally. Stir in undrained tomatoes, tomato sauce, Italian seasoning and salt. Bring to a boil. Reduce heat and simmer 3 minutes, stirring occasionally. Meanwhile, combine ricotta cheese, 1/4 cup Parmesan cheese and pepper in small bowl; set aside.
2. Spread 3/4 cup sauce mixture over bottom of slow cooker. Layer 3 lasagna noodles over sauce mixture, breaking noodles to fit. Top with 3/4 cup sauce mixture, ricotta cheese mixture and 1 cup Italian cheese. Top with 3/4 cup sauce mixture and remaining 3 lasagna noodles, breaking to fit. Spoon remaining sauce mixture over noodles.
3. Cover; cook on LOW 5 hours or until noodles are tender. Sprinkle with remaining 1/2 cup Italian cheese and remaining 1/4 cup Parmesan cheese. Cover; let stand 5 minutes or until cheese melts.
Recipe by About A Mom at https://www.aboutamom.com/slow-cooker-lasagna/